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IPv6 Unicast Address Assignment Considerations

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 5375.
Authors Chip Popoviciu , Christian Hahn , Olaf Bonness , Gunter Van de Velde , Tim Chown
Last updated 2020-01-21 (Latest revision 2008-09-22)
Replaces draft-vandevelde-v6ops-addcon
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Informational
Additional resources Mailing list discussion
Stream WG state (None)
Document shepherd (None)
IESG IESG state Became RFC 5375 (Informational)
Action Holders
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Ron Bonica
Send notices to (None)
IPv6 Operations                                          G. Van de Velde
Internet-Draft                                              C. Popoviciu
Intended status: Informational                             Cisco Systems
Expires: March 26, 2009                                         T. Chown
                                               University of Southampton
                                                              O. Bonness
                                                                 C. Hahn
                                      T-Systems Enterprise Services GmbH
                                                      September 22, 2008

             IPv6 Unicast Address Assignment Considerations

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on March 26, 2009.


   One fundamental aspect of any IP communications infrastructure is its
   addressing plan.  With its new address architecture and allocation
   policies, the introduction of IPv6 into a network means that network
   designers and operators need to reconsider their existing approaches
   to network addressing.  Lack of guidelines on handling this aspect of
   network design could slow down the deployment and integration of
   IPv6.  This document aims to provide the information and

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   recommendations relevant to planning the addressing aspects of IPv6
   deployments.  The document also provides IPv6 addressing case studies
   for both an enterprise and an ISP network.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Network Level Addressing Design Considerations . . . . . . . .  5
     2.1.  Globally Unique Addresses  . . . . . . . . . . . . . . . .  5
     2.2.  Unique Local IPv6 Addresses  . . . . . . . . . . . . . . .  5
     2.3.  6Bone Address Space  . . . . . . . . . . . . . . . . . . .  7
     2.4.  Network Level Design Considerations  . . . . . . . . . . .  7
       2.4.1.  Sizing the Network Allocation  . . . . . . . . . . . .  8
       2.4.2.  Address Space Conservation . . . . . . . . . . . . . .  9
   3.  Subnet Prefix Considerations . . . . . . . . . . . . . . . . .  9
     3.1.  Considerations for /64 Prefixes  . . . . . . . . . . . . . 10
     3.2.  Allocation of the IID of an IPv6 Address . . . . . . . . . 10
       3.2.1.  Automatic EUI-64 Format Option . . . . . . . . . . . . 11
       3.2.2.  Using Privacy Extensions . . . . . . . . . . . . . . . 11
       3.2.3.  Manual/Dynamic Assignment Option . . . . . . . . . . . 11
     3.3.  IANA Considerations  . . . . . . . . . . . . . . . . . . . 12
     3.4.  Security Considerations  . . . . . . . . . . . . . . . . . 12
     3.5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . 12
   4.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     4.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Case Studies  . . . . . . . . . . . . . . . . . . . . 15
     A.1.  Enterprise Considerations  . . . . . . . . . . . . . . . . 15
       A.1.1.  Obtaining General IPv6 Network Prefixes  . . . . . . . 16
       A.1.2.  Forming an Address (subnet) Allocation Plan  . . . . . 16
       A.1.3.  Other Considerations . . . . . . . . . . . . . . . . . 17
       A.1.4.  Node Configuration Considerations  . . . . . . . . . . 18
     A.2.  Service Provider Considerations  . . . . . . . . . . . . . 18
       A.2.1.  Investigation of objective Requirements for an
               IPv6  addressing schema of a Service Provider  . . . . 19
       A.2.2.  Exemplary IPv6 Address Allocation Plan for a
               Service Provider . . . . . . . . . . . . . . . . . . . 22
       A.2.3.  Additional Remarks . . . . . . . . . . . . . . . . . . 26
   Appendix B.  Considerations for Subnet Prefixes Different then
                /64 . . . . . . . . . . . . . . . . . . . . . . . . . 29
     B.1.  Considerations for Subnet Prefixes Shorter then /64  . . . 29
     B.2.  Considerations for Subnet Prefixes Longer then /64 . . . . 29
       B.2.1.  /126 Addresses . . . . . . . . . . . . . . . . . . . . 29
       B.2.2.  /127 Addresses . . . . . . . . . . . . . . . . . . . . 29
       B.2.3.  /128 Addresses . . . . . . . . . . . . . . . . . . . . 29
       B.2.4.  EUI-64 'u' and 'g' bits  . . . . . . . . . . . . . . . 30
       B.2.5.  Anycast Addresses  . . . . . . . . . . . . . . . . . . 31

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       B.2.6.  Addresses Used by Embedded-RP (RFC3956)  . . . . . . . 32
       B.2.7.  ISATAP Addresses . . . . . . . . . . . . . . . . . . . 33
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
   Intellectual Property and Copyright Statements . . . . . . . . . . 35

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1.  Introduction

   The Internet Protocol Version 6 (IPv6) Addressing Architecture
   [RFC4291] defines three main types of addresses: unicast, anycast and
   multicast.  This document focuses on unicast addresses, for which
   there are currently two principal allocated types: Globally Unique
   Addresses [RFC3587] ('globals') and Unique Local IPv6 Addresses
   [RFC4193] (ULAs).  In addition until recently there has been
   'experimental' 6bone address space [RFC3701], though its use has been
   deprecated since June 2006 [RFC3701].

   The document covers aspects that should be considered during IPv6
   deployment for the design and planning of an addressing scheme for an
   IPv6 network.  The network's IPv6 addressing plan may be for an IPv6-
   only network, or for a dual-stack infrastructure where some or all
   devices have addresses in both protocols.  These considerations will
   help an IPv6 network designer to efficiently and prudently assign the
   IPv6 address space that has been allocated to their organization.

   The address assignment considerations are analyzed separately for the
   two major components of the IPv6 unicast addresses, namely 'Network
   Level Addressing' (the allocation of subnets) and the 'interface-id'
   (the identification of the interface within a subnet).  Thus the
   document includes a discussion of aspects of address assignment to
   nodes and interfaces in an IPv6 network.  Finally the document
   provides two examples of deployed address plans in a service provider
   (ISP) and an enterprise network.

   Parts of this document highlight the differences that an experienced
   IPv4 network designer should consider when planning an IPv6
   deployment, for example:

   o  IPv6 devices will more likely be multi-addressed in comparison
      with their IPv4 counterparts
   o  The practically unlimited size of an IPv6 subnet (2^64 bits)
      reduces the requirement to size subnets to device counts for the
      purposes of (IPv4) address conservation
   o  The implications of the vastly increased subnet size on the threat
      of address-based host scanning and other scanning techniques, as
      discussed in [RFC5157].

   We do not discuss here how a site or ISP should proceed with
   acquiring its globally routable IPv6 address prefix.  In each case
   the prefix received is either provider assigned (PA) or provider
   independent (PI).

   We do not discuss PI policy here.  The observations and
   recommendations of this text are largely independent of the PA or PI

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   nature of the address block being used.  At this time we assume that
   most commonly an IPv6 network which changes provider will need to
   undergo a renumbering process, as described in [RFC4192].  A separate
   document [THINKABOUT] makes recommendations to ease the IPv6
   renumbering process.

   This document does not discuss implementation aspects related to the
   transition between the ULA addresses and the now obsoleted site-local
   addresses.  Some implementations know about Site-local addresses even
   though they are deprecated, and do not know about ULAs - even though
   they represent current specification.  As result transitioning
   between these types of addresses may cause difficulties.

2.  Network Level Addressing Design Considerations

   This section discusses the kind of IPv6 addresses used at the network
   level for the IPv6 infrastructure.  The kind of addresses that can be
   considered are Globally Unique Addresses and ULAs.  We also comment
   here on the deprecated 6bone address space.

2.1.  Globally Unique Addresses

   The most commonly used unicast addresses will be Globally Unique
   Addresses ('globals').  No significant considerations are necessary
   if the organization has an address space assignment and a single
   prefix is deployed through a single upstream provider.

   However, a multihomed site may deploy addresses from two or more
   Service Provider assigned IPv6 address ranges.  Here, the network
   Administrator must have awareness on where and how these ranges are
   used on the multihomed infrastructure environment.  The nature of the
   usage of multiple prefixes may depend on the reason for multihoming
   (e.g. resilience failover, load balancing, policy-based routing, or
   multihoming during an IPv6 renumbering event).  IPv6 introduces
   improved support for multi-addressed hosts through the IPv6 default
   address selection methods described in RFC3484 [RFC3484].  A
   multihomed host may thus have two or more addresses, one per prefix
   (provider), and select source and destination addresses to use as
   described in that RFC.  However multihoming also has some operational
   and administrative burdens besides chosing multiple addresses per
   interface [RFC4219][RFC4218].

2.2.  Unique Local IPv6 Addresses

   ULAs have replaced the originally conceived Site Local addresses in
   the IPv6 addressing architecture, for reasons described in [RFC3879].
   ULAs improve on site locals by offering a high probability of the

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   global uniqueness of the prefix used, which can be beneficial in the
   case of (deliberate or accidental) leakage, or where networks are
   merged.  ULAs are akin to the private address space [RFC1918]
   assigned for IPv4 networks, except that in IPv6 networks we may
   expect to see ULAs used alongside global addresses, with ULAs used
   internally and globals used externally.  Thus use of ULAs does not
   imply use of NAT for IPv6.

   The ULA address range allows network administrators to deploy IPv6
   addresses on their network without asking for a globally unique
   registered IPv6 address range.  A ULA prefix is 48 bits, i.e. a /48,
   the same as the currently recommended allocation for a site from the
   globally routable IPv6 address space [RFC3177].

   A site willing to use ULA address space can have either (a) multiple
   /48 prefixes (e.g. a /44) and wishes to use ULAs, or (b) has one /48
   and wishes to use ULAs or (c) a site has a less-than-/48 prefix (e.g.
   a /56 or /64) and wishes to use ULAs.  In all above cases the ULA
   addresses can be randomly chosen according the principles specified
   in [RFC4193].  However, in case (a) the use of randomly chosen ULA
   addresses will provide suboptimal aggregation capabilities.

   ULAs provide the means to deploy a fixed addressing scheme that is
   not affected by a change in service provider and the corresponding PA
   global addresses.  Internal operation of the network is thus
   unaffected during renumbering events.  Nevertheless, this type of
   address must be used with caution.

   A site using ULAs may or may not also deploy global addresses.  In an
   isolated network ULAs may be deployed on their own.  In a connected
   network, that also deploys global addresses, both may be deployed,
   such that hosts become multiaddressed (one global and one ULA
   address) and the IPv6 default address selection algorithm will pick
   the appropriate source and destination addresses to use, e.g.  ULAs
   will be selected where both the source and destination hosts have ULA
   addresses.  Because a ULA and a global site prefix are both /48
   length, an administrator can choose to use the same subnetting (and
   host addressing) plan for both prefixes.

   As an example of the problems ULAs may cause, when using IPv6
   multicast within the network, the IPv6 default address selection
   algorithm prefers the ULA address as the source address for the IPv6
   multicast streams.  This is NOT a valid option when sending an IPv6
   multicast stream to the IPv6 Internet for two reasons.  For one,
   these addresses are not globally routable so Reverse Path Forwarding
   checks for such traffic will fail outside the internal network.  The
   other reason is that the traffic will likely not cross the network
   boundary due to multicast domain control and perimeter security

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   In principle ULAs allow easier network mergers than RFC1918 addresses
   do for IPv4 because ULA prefixes have a high probability of
   uniqueness, if the prefix is chosen as described in the RFC.

2.3.  6Bone Address Space

   The 6Bone address space was used before the Regional Internet
   Registries (RIRs) started to distribute 'production' IPv6 prefixes.
   The 6Bone prefixes have a common first 16 bits in the IPv6 Prefix of
   3FFE::/16.  This address range is deprecated as of 6th June 2006
   [RFC3701] and must not be used on any new IPv6 network deployments.
   Sites using 6bone address space should renumber to production address
   space using procedures as defined in [RFC4192].

2.4.  Network Level Design Considerations

   IPv6 provides network administrators with a significantly larger
   address space, enabling them to be very creative in how they can
   define logical and practical address plans.  The subnetting of
   assigned prefixes can be done based on various logical schemes that
   involve factors such as:
   o  Using existing systems
      *  translate the existing subnet number into IPv6 subnet id
      *  translate the VLAN id into IPv6 subnet id
   o  Redesign
      *  allocate according to your need
   o  Aggregation
      *  Geographical Boundaries - by assigning a common prefix to all
         subnets within a geographical area
      *  Organizational Boundaries - by assigning a common prefix to an
         entire organization or group within a corporate infrastructure
      *  Service Type - by reserving certain prefixes for predefined
         services such as: VoIP, Content Distribution, wireless
         services, Internet Access, Security areas etc.  This type of
         addressing may create dependencies on IP addresses that can
         make renumbering harder if the nodes or interfaces supporting
         those services on the network are sparse within the topology.
   Such logical addressing plans have the potential to simplify network
   operations and service offerings, and to simplify network management
   and troubleshooting.  A very large network would also have no need to
   consider using private address space for its infrastructure devices,
   simplifying network management.

   The network designer must however keep in mind several factors when
   developing these new addressing schemes for networks with and without
   global connectivity:

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   o  Prefix Aggregation - The larger IPv6 addresses can lead to larger
      routing tables unless network designers are actively pursuing
      aggregation.  While prefix aggregation will be enforced by the
      service provider, it is beneficial for the individual
      organizations to observe the same principles in their network
      design process
   o  Network growth - The allocation mechanism for flexible growth of a
      network prefix, documented in RFC3531 [RFC3531] can be used to
      allow the network infrastructure to grow and be numbered in a way
      that is likely to preserve aggregation (the plan leaves 'holes'
      for growth)
   o  ULA usage in large networks - Networks which have a large number
      of 'sites' that each deploy a ULA prefix which will by default be
      a 'random' /48 under fc00::/7 will have no aggregation of those
      prefixes.  Thus the end result may be cumbersome because the
      network will have large amounts of non-aggregated ULA prefixes.
      However, there is no rule to disallow large networks to use a
      single ULA prefix for all 'sites', as a ULA still provides 16 bits
      for subnetting to be used internally
   o  It is possible that as registry policies evolve, a small site may
      experience an increase in prefix length when renumbering, e.g.
      from /48 to /56.  For this reason, the best practice is number
      subnets compactly rather than sparsely, and to use low-order bits
      as much as possible when numbering subnets.  In other words, even
      if a /48 is allocated, act as though only a /56 is available.
      Clearly, this advice does not apply to large sites and enterprises
      that have an intrinsic need for a /48 prefix.
   o  A small site may want to enable routing amongst interfaces
      connected to a gateway device.  For example, a residential gateway
      which receives a /48, is situated in a home with multiple LANs of
      different media types (sensor network, wired, wifi, etc.), or has
      a need for traffic segmentation (home, work, kids, etc.) and could
      benefit greatly from multiple subnets and routing in IPv6.
      Ideally, residential networks would be given an address range of a
      /48 or /56 [reference2] such that multiple /64 subnets could be
      used within the residence.

2.4.1.  Sizing the Network Allocation

   We do not discuss here how a network designer sizes their application
   for address space.  By default a site will receive a /48 prefix
   [RFC3177] , however different RIR service regions policies may
   suggest alternative default assignments or let the ISPs to decide on
   what they believe is more appropriate for their specific case [ARIN].
   The default provider allocation via the RIRs is currently a /32
   [reference2].  These allocations are indicators for a first
   allocation for a network.  Different sizes may be obtained based on
   the anticipated address usage [reference2].  There are examples of

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   allocations as large as /19 having been made from RIRs to providers
   at the time of writing.

2.4.2.  Address Space Conservation

   Despite the large IPv6 address space which enables easier subnetting,
   it still is important to ensure an efficient use of this resource.
   Some addressing schemes, while facilitating aggregation and
   management, could lead to significant numbers of addresses being
   unused.  Address conservation requirements are less stringent in IPv6
   but they should still be observed.

   The proposed Host-Density (HD) [RFC3194] value for IPv6 is 0.94
   compared to the current value of 0.96 for IPv4.  Note that for IPv6
   HD is calculated for sites (e.g. on a basis of /48), instead of based
   on addresses like with IPv4.

3.  Subnet Prefix Considerations

   An important part of an IPv4 addressing plan is deciding the length
   of each subnet prefix.  Unlike in IPv4, the IPv6 addressing
   architecture [RFC4291] specifies that all subnets using Globally
   Unique Addresses and ULAs always have the same prefix length of 64
   bits.  (This applies also to the deprecated 6Bone and Site Local

   The only exception to this rule are special addresses starting with
   the binary value 000, such as IPv4-Compatible IPv6 Addresses.  These
   exceptions are largely beyond the scope of this document.

   Using a subnet prefix length other than a /64 will break many
   features of IPv6, amongst other things Neighbor Discovery (ND),
   Secure Neighborship Discovery (SEND) [RFC3971], privacy extensions
   [RFC4941], parts of Mobile IPv6 [RFC4866], PIM-SM with Embedded-RP
   [RFC3956], and SHIM6 [SHIM6].  A number of other features currently
   in development, or being proposed, also rely on /64 subnet prefixes.

   Nevertheless, many IPv6 implementations do not prevent the
   administrator from configuring a subnet prefix length shorter or
   longer than 64 bits.  Using subnet prefixes shorter than /64 would
   rarely be useful; see Appendix B.1 for discussion.

   However, some network administrators have used prefixes longer than
   /64 for links connecting routers, usually just two routers on a
   point-to-point link.  On links where all the addresses are assigned
   by manual configuration, and all nodes on the link are routers (not
   end hosts) that are known by the network administrators do not need

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   any of the IPv6 features that rely on /64 subnet prefixes, this can
   work.  Using subnet prefixes longer than /64 are not recommended for
   general use, and using them for links containing end hosts would be
   an especially bad idea, as it is difficult to predict what IPv6
   features the hosts will use in the future.

   Appendix B.2 describes some practical considerations that need to be
   taken into account when using prefixes longer than /64 in limited
   cases.  In particular, a number of IPv6 features use interface
   identifiers that have a special form (such as a certain fixed value
   in some bit positions).  When using prefixes longer than /64, it is
   prudent to avoid certain subnet prefix values so that nodes who
   assume that the prefix is /64 will not incorrectly identify the
   addresses in that subnet as having a special form.  Appendix B.2
   describes the subnet prefix values that are currently believed to be
   potentially problematic; however, the list is not exhaustive and can
   be expected to grow in the future.

   Using /64 subnets is strongly recommended, also for links connecting
   only routers.  A deployment compliant with the current IPv6
   specifications cannot use other prefix lengths.  However, the V6OPS
   WG believes that despite the drawbacks (and a potentially expensive
   network redesign, if IPv6 features relying on /64 subnets are needed
   in the future), that some networks administrators will use prefixes
   longer than /64.

3.1.  Considerations for /64 Prefixes

   Based on RFC3177 [RFC3177], 64 bits is the prescribed subnet prefix
   length to allocate to interfaces and nodes.

   When using a /64 subnet length, the address assignment for these
   addresses can be made either by manual configuration, by a stateful
   Host Configuration Protocol [RFC3315] [RFC3736] or by stateless
   autoconfiguration [RFC4862].

   Note that RFC3177 strongly prescribes 64 bit subnets for general
   usage, and that stateless autoconfiguration option is only defined
   for 64 bit subnets.  While in theory it might be possible that some
   future autoconfiguration mechanisms would allow longer than 64 bit
   prefix lengths to be used, the use of such prefixes is not
   recommended at this time.

3.2.  Allocation of the IID of an IPv6 Address

   In order to have a complete IPv6 address, an interface must be
   associated a prefix and an Interface Identifier (IID).  Section 3 of
   this document analyzed the prefix selection considerations.  This

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   section discusses the elements that should be considered when
   assigning the IID portion of the IPv6 address.

   There are various ways to allocate an IPv6 address to a device or
   interface.  The option with the least amount of caveats for the
   network administrator is that of EUI-64 [RFC4862] based addresses.
   For the manual or dynamic options, the overlap with well known IPv6
   addresses should be avoided.

3.2.1.  Automatic EUI-64 Format Option

   When using this method the network administrator has to allocate a
   valid 64 bit subnet prefix.  The EUI-64 [RFC4862] allocation
   procedure can from that moment onward assign the remaining 64 IID
   bits in a stateless manner.  All the considerations for selecting a
   valid IID have been incorporated in the EUI-64 methodology.

3.2.2.  Using Privacy Extensions

   The main purpose of IIDs generated based on RFC4941 [RFC4941] is to
   provide privacy to the entity using this address.  While there are no
   particular constraints in the usage of these addresses as defined in
   [RFC4941] there are some implications to be aware of when using
   privacy addresses as documented in section 4 of RFC4941 [RFC4941]

3.2.3.  Manual/Dynamic Assignment Option

   This section discusses those IID allocations that are not implemented
   through stateless address configuration (Section 4.1).  They are
   applicable regardless of the prefix length used on the link.  It is
   out of scope for this section to discuss the various assignment
   methods (e.g. manual configuration, DHCPv6, etc).

   In this situation the actual allocation is done by human intervention
   and consideration needs to be given to the complete IPv6 address so
   that it does not result in overlaps with any of the well known IPv6
   o  Subnet Router Anycast Address (Appendix B.2.5.1.)
   o  Reserved Subnet Anycast Address (Appendix B.2.5.2.)
   o  Addresses used by Embedded-RP (Appendix B.2.6.)
   o  ISATAP Addresses (Appendix B.2.7.)

   When using an address assigned by human intervention it is
   recommended to choose IPv6 addresses which are not obvious to guess
   and/or avoid any IPv6 addresses that embed IPv4 addresses used in the
   current infrastructure.  Following these two recommendations will
   make it more difficult for malicious third parties to guess targets
   for attack, and thus reduce security threats to a certain extent.

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3.3.  IANA Considerations

   There are no extra IANA consideration for this document.

3.4.  Security Considerations

   This document doesn't add any new security considerations that aren't
   already outlined in the security considerations of the references.

   It must be noted that using subnet prefixes other than /64 breaks
   security mechanisms such as Cryptographically Generated Addresses
   (CGAs) and Hash Based Addresses (HBAs), and thus makes it impossible
   to use protocols that depend on them.

3.5.  Acknowledgements

   Constructive feedback and contributions have been received during
   IESG review cycle and from Marla Azinger, Stig Venaas, Pekka Savola,
   John Spence, Patrick Grossetete, Carlos Garcia Braschi, Brian
   Carpenter, Mark Smith, Janos Mohacsi, Jim Bound, Fred Templin, Ginny
   Listman, Salman Assadullah, Krishnan Thirukonda and the IESG.

4.  References

4.1.  Normative References

4.2.  Informative References

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC2526]  Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
              Addresses", RFC 2526, March 1999.

   [RFC3021]  Retana, A., White, R., Fuller, V., and D. McPherson,
              "Using 31-Bit Prefixes on IPv4 Point-to-Point Links",
              RFC 3021, December 2000.

   [RFC3053]  Durand, A., Fasano, P., Guardini, I., and D. Lento, "IPv6
              Tunnel Broker", RFC 3053, January 2001.

   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
              via IPv4 Clouds", RFC 3056, February 2001.

   [RFC3177]  IAB and IESG, "IAB/IESG Recommendations on IPv6 Address
              Allocations to Sites", RFC 3177, September 2001.

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   [RFC3180]  Meyer, D. and P. Lothberg, "GLOP Addressing in 233/8",
              BCP 53, RFC 3180, September 2001.

   [RFC3194]  Durand, A. and C. Huitema, "The H-Density Ratio for
              Address Assignment Efficiency An Update on the H ratio",
              RFC 3194, November 2001.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

   [RFC3531]  Blanchet, M., "A Flexible Method for Managing the
              Assignment of Bits of an IPv6 Address Block", RFC 3531,
              April 2003.

   [RFC3587]  Hinden, R., Deering, S., and E. Nordmark, "IPv6 Global
              Unicast Address Format", RFC 3587, August 2003.

   [RFC3627]  Savola, P., "Use of /127 Prefix Length Between Routers
              Considered Harmful", RFC 3627, September 2003.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.

   [RFC3701]  Fink, R. and R. Hinden, "6bone (IPv6 Testing Address
              Allocation) Phaseout", RFC 3701, March 2004.

   [RFC3736]  Droms, R., "Stateless Dynamic Host Configuration Protocol
              (DHCP) Service for IPv6", RFC 3736, April 2004.

   [RFC3879]  Huitema, C. and B. Carpenter, "Deprecating Site Local
              Addresses", RFC 3879, September 2004.

   [RFC3956]  Savola, P. and B. Haberman, "Embedding the Rendezvous
              Point (RP) Address in an IPv6 Multicast Address",
              RFC 3956, November 2004.

   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC4192]  Baker, F., Lear, E., and R. Droms, "Procedures for
              Renumbering an IPv6 Network without a Flag Day", RFC 4192,
              September 2005.

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   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.

   [RFC4218]  Nordmark, E. and T. Li, "Threats Relating to IPv6
              Multihoming Solutions", RFC 4218, October 2005.

   [RFC4219]  Lear, E., "Things Multihoming in IPv6 (MULTI6) Developers
              Should Think About", RFC 4219, October 2005.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4477]  Chown, T., Venaas, S., and C. Strauf, "Dynamic Host
              Configuration Protocol (DHCP): IPv4 and IPv6 Dual-Stack
              Issues", RFC 4477, May 2006.

   [RFC4798]  De Clercq, J., Ooms, D., Prevost, S., and F. Le Faucheur,
              "Connecting IPv6 Islands over IPv4 MPLS Using IPv6
              Provider Edge Routers (6PE)", RFC 4798, February 2007.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

   [RFC4866]  Arkko, J., Vogt, C., and W. Haddad, "Enhanced Route
              Optimization for Mobile IPv6", RFC 4866, May 2007.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, September 2007.

   [RFC5214]  Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
              Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
              March 2008.

   [RFC5157]  Chown, T., "IPv6 Implications for Network Scanning",
              RFC 5157, March 2008.

   [SHIM6]    IETF,

   [ARIN]     ARIN, "".

              APNIC, ARIN, RIPE NCC, "
              ipv6policy.html", July 2007.

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              APNIC, ARIN, RIPE NCC,
              "", July 2007.

              ARIN, "",
              March 2008.

              March 2007.

              LACNIC, "".

              AFRINIC, "
              afpol-v6200407-000.htm", March 2004.

              Chown, T., Thompson, M., Ford, A., and S. Venaas, "Things
              to think about when Renumbering an IPv6 network
              March 2007.

Appendix A.  Case Studies

   This appendix contains two case studies for IPv6 addressing schemas
   that have been based on the statements and considerations of this
   draft.  These case studies illustrate how this draft has been used in
   two specific network scenarios.  The case studies may serve as basic
   considerations for an administrator who designs the IPv6 addressing
   schema for an enterprise or ISP network, but are not intended to
   serve as general design proposal for every kind of IPv6 network.  All
   subnet sizes used in this appendix are for practical visualization
   and do not dictate RIR policy.

A.1.  Enterprise Considerations

   In this section one considers a case study of a campus network that
   is deploying IPv6 in parallel with existing IPv4 protocols in a dual-
   stack environment.  The specific example is the University of
   Southampton (UK), focusing on a large department within that network.
   The deployment currently spans around 1,000 hosts and over 1,500

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A.1.1.  Obtaining General IPv6 Network Prefixes

   In the case of a campus network, the site will typically take its
   connectivity from its National Research and Education Network (NREN).
   Southampton connects to JANET, the UK academic network, via its local
   regional network LeNSE.  JANET currently has a /32 allocation from
   RIPE NCC.  The current recommended practice is for sites to receive a
   /48 allocation, and on this basis Southampton has received such a
   prefix for its own use.  The regional network also uses its own
   allocation from the NREN provider.

   No ULA addressing is used on site.  The campus is not multihomed
   (JANET is the sole provider), nor does it expect to change service
   provider, and thus does not plan to use ULAs for the (perceived)
   benefit of easing network renumbering.  Indeed, the campus has
   renumbered following the aforementioned renumbering procedure
   [RFC4192] on two occasions, and this has proven adequate (with
   provisos documented in [THINKABOUT].  The campus do not see any need
   to deploy ULAs for in or out of band network management; there are
   enough IPv6 prefixes available in the site allocation for the
   infrastructure.  In some cases, use of private IP address space in
   IPv4 creates problems, so University of Southampton believe that the
   availability of ample global IPv6 address space for infrastructure
   may be a benefit for many sites.

   No 6bone addressing is used on site any more.  Since the 6bone
   phaseout of June 2006 [RFC3701] most transit ISPs have begun
   filtering attempted use of such prefixes.

   Southampton does participate in global and organization scope IPv6
   multicast networks.  Multicast address allocations are not discussed
   here as they are not in scope for the document.  It is noted that
   IPv6 has advantages for multicast group address allocation.  In IPv4
   a site needs to use techniques like GLOP [RFC3180] to pick a globally
   unique multicast group to use.  This is problematic if the site does
   not use Border Gateway Protocol (BGP) [RFC4271] and have an
   Autonomous System Number (ASN).  In IPv6 unicast-prefix-based IPv6
   multicast addresses empower a site to pick a globally unique group
   address based on its unicast own site or link prefix.  Embedded RP is
   also in use, is seen as a potential advantage for IPv6 and multicast,
   and has been tested successfully across providers between sites
   (including paths to/from the US and UK).

A.1.2.  Forming an Address (subnet) Allocation Plan

   The campus has a /16 prefix for IPv4 use; in principle 256 subnets of
   256 addresses.  In reality the subnetting is muddier, because of
   concerns of IPv4 address conservation; subnets are sized to the hosts

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   within them, e.g. a /26 IPv4 prefix is used if a subnet has 35 hosts
   in it.  While this is efficient, it increases management burden when
   physical deployments change, and IPv4 subnets require resizing (up or
   down), even with DHCP in use.

   The /48 IPv6 prefix is considerably larger than the IPv4 allocation
   already in place at the site.  It is loosely equivalent to a 'Class
   A' IPv4 prefix in that it has 2^16 (over 65,000) subnets, but has an
   effectively unlimited subnet address size (2^64) compared to 256 in
   the IPv4 equivalent.  The increased subnet size means that /64 IPv6
   prefixes can be used on all subnets, without any requirement to
   resize them at a later date.  The increased subnet volume allows
   subnets to be allocated more generously to schools and departments in
   the campus.  While address conservation is still important, it is no
   longer an impediment on network management.  Rather, address (subnet)
   allocation is more about embracing the available address space and
   planning for future expansion.

   In a dual-stack network, it was chosen to deploy our IP subnets
   congruently for IPv4 and IPv6.  This is because the systems are still
   in the same administrative domains and the same geography.  It is not
   expected to have IPv6-only subnets in production use for a while yet,
   outside the test beds and some early Mobile IPv6 trials.  With
   congruent addressing, our firewall policies are also aligned for IPv4
   and IPv6 traffic at the site border.

   The subnet allocation plan required a division of the address space
   per school or department.  Here a /56 was allocated to the school
   level of the university; there are around 30 schools currently.  A
   /56 of IPv6 address space equates to 256 /64 size subnet allocations.
   Further /56 allocations were made for central IT infrastructure, for
   the network infrastructure and the server side systems.

A.1.3.  Other Considerations

   The network uses a Demilitarized Zone (DMZ) topology for some level
   of protection of 'public' systems.  Again, this topology is congruent
   with the IPv4 network.

   There are no specific transition methods deployed internally to the
   campus; everything is using the conventional dual-stack approach.
   There is no use of ISATAP [RFC5214] for example.

   For the Mobile IPv6 early trials there is one allocated prefix for
   Home Agent (HA) use.  However there has been no detailed
   consideration yet how Mobile IPv6 usage may grow, and whether more or
   even every subnet will require HA support.

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   The university operates a tunnel broker [RFC3053] service on behalf
   of UKERNA for JANET sites.  This uses separate address space from
   JANET, not our university site allocation.

A.1.4.  Node Configuration Considerations

   Currently stateless autoconfiguration is used on most subnets for
   IPv6 hosts.  There is no DHCPv6 service deployed yet, beyond tests of
   early code releases.  It is planned to deploy DHCPv6 for address
   assignment when robust client and server code is available (at the
   time of writing the potential for this looks good, e.g. via the ISC
   implementation).  University of Southampton is also investigating a
   common integrated DHCP/DNS management platform, even if the servers
   themselves are not co-located, including integrated DHCPv4 and DHCPv6
   server configuration, as discussed in [RFC4477].  Currently clients
   with statelessly autoconfigured addresses are added to the DNS
   manually, though dynamic DNS is an option.  The network
   administrators would prefer the use of DHCP because they believe it
   gives them more management control.

   Regarding the implications of the larger IPv6 subnet address space on
   scanning attacks [RFC5157], it is noted that all the hosts are dual-
   stack, and thus are potentially exposed over both protocols anyway.
   All addresses or published in DNS, and hence do not operate a two
   faced DNS.

   There is internal usage of RFC4941 privacy addresses [RFC4941]
   currently (certain platforms currently ship with it on by default),
   but may desire to administratively disable this (perhaps via DHCP) to
   ease management complexity.  However, it is desired to determine the
   feasibility of this on all systems, e.g. for guests on wireless LAN
   or other user-maintained systems.  Network management and monitoring
   should be simpler without RFC4941 in operation, in terms of
   identifying which physical hosts are using which addresses.  Note
   that RFC4941 is only an issue for outbound connections, and that
   there is potential to assign privacy addresses via DHCPv6.

   Manually configured server addresses are used to avoid address
   changes based upon change of network adaptor.  With IPv6 you can
   choose to pick ::53 for a DNS server, or can pick 'random' addresses
   for obfuscation, though that's not an issue for publicly advertised
   addresses (dns, mx, web, etc).

A.2.  Service Provider Considerations

   In this section an IPv6 addressing schema is sketched that could
   serve as an example for an Internet Service Provider.

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   Sub-section A.2.1 starts with some thoughts regarding objective
   requirements of such an addressing schema and derives a few general
   rules of thumb that have to be kept in mind when designing an ISP
   IPv6 addressing plan.

   Sub-section A.2.2 illustrates these findings of A.2.1 with an
   exemplary IPv6 addressing schema for an MPLS-based ISP offering
   Internet Services as well as Network Access services to several
   millions of customers.

A.2.1.  Investigation of objective Requirements for an IPv6  addressing
        schema of a Service Provider

   The first step of the IPv6 addressing plan design for a Service
   provider should identify all technical, operational, political and
   business requirements that have to be satisfied by the services
   supported by this addressing schema.

   According to the different technical constraints and business models
   as well as the different weights of these requirements (from the
   point of view of the corresponding Service Provider) it is very
   likely that different addressing schemas will be developed and
   deployed by different ISPs.  Nevertheless the addressing schema of
   sub-section A.2.2 is one possible example.

   For this document it is assumed that our exemplary ISP has to fulfill
   several roles for its customers as there are:

   o  Local Internet Registry
   o  Network Access Provider
   o  Internet Service Provider

A.2.1.1.  Recommendations for an IPv6 Addressing Schema from the LIR
          Perspective of the Service Provider

   In their role as Local Internet Registry (LIR) the Service Providers
   have to care about the policy constraints of the RIRs and the
   standards of the IETF regarding IPv6 addressing.  In this context,
   the following basic recommendations have to be considered and should
   be satisfied by the IPv6 address allocation plan of a Service
   o  As recommended in RFC 3177 [RFC3177] and in several RIR policies
      "Common" customers sites (normally private customers) should
      receive a /48 prefix from the aggregate of the Service Provider.
      (Note: The addressing plan must be flexible enough and take into
      account the possible change of the minimum allocation size for end
      users currently under definition by the RIRs.)

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   o  "Big customers" (like big enterprises, governmental agencies etc.)
      may receive shorter prefixes according to their needs when this
      need could be documented and justified to the RIR.
   o  The IPv6 address allocation schema has to be able to meet the HD-
      ratio that is proposed for IPv6.  This requirement corresponds to
      the demand for an efficient usage of the IPv6 address aggregate by
      the Service Provider.  (Note: The currently valid IPv6 HD-ratio of
      0.94 means an effective usage of about 31% of a /20 prefix of the
      Service Provider on the basis of /48 assignments.)
   o  All assignments to customers have to be documented and stored into
      a database that can also be queried by the RIR.
   o  The LIR has to make available means for supporting the reverse DNS
      mapping of the customer prefixes.
   o  IPv6 Address Allocation and Assignment Policies can be found at
      RIRs and are similar in many aspects:
      [reference2][reference3][reference4] [reference5][reference6]

A.2.1.2.  IPv6 Addressing Schema Recommendations from the ISP
          Perspective of the Service Provider

   From ISP perspective the following basic requirements could be
   o  The IPv6 address allocation schema must be able to realize a
      maximal aggregation of all IPv6 address delegations to customers
      into the address aggregate of the Service Provider.  Only this
      provider aggregate will be routed and injected into the global
      routing table (DFZ).  This strong aggregation keeps the routing
      tables of the DFZ small and eases filtering and access control
      very much.
   o  The IPv6 addressing schema of the SP should contain optimal
      flexibility since the infrastructure of the SP will change over
      the time with new customers, transport technologies and business
      cases.  The requirement of optimal flexibility is contrary to the
      recommendation of strong IPv6 address aggregation and efficient
      address usage, but at this point each SP has to decide which of
      these requirements to prioritize.
   o  Keeping the multilevel network hierarchy of an ISP in mind, due to
      addressing efficiency reasons not all hierarchy levels can and
      should be mapped into the IPv6 addressing schema of an ISP.
      Sometimes it is much better to implement a more "flat" addressing
      for the ISP network than to loose big chunks of the IPv6 address
      aggregate in addressing each level of network hierarchy.  (Note:
      In special cases it is even recommendable for really "small" ISPs
      to design and implement a totally flat IPv6 addressing schema
      without any level of hierarchy.)
   o  Besides that a decoupling of provider network addressing and
      customer addressing is recommended.  (Note: A strong aggregation
      e.g. on POP, aggregation router or Label Edge Router (LER) level

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      limits the numbers of customer routes that are visible within the
      ISP network but brings also down the efficiency of the IPv6
      addressing schema.  That's why each ISP has to decide how many
      internal aggregation levels it wants to deploy.)

A.2.1.3.  IPv6 Addressing Schema Recommendations from the Network Access
          provider Perspective of the Service Provider

   As already done for the LIR and the ISP roles of the SP it is also
   necessary to identify requirements that come from its Network Access
   Provider role.  Some of the basic requirements are:
   o  The IPv6 addressing schema of the SP must be chosen in a way that
      it can handle new requirements that are triggered from customer
      side.  This can be for instance the growing needs of the customers
      regarding IPv6 addresses as well as customer driven modifications
      within the access network topology (e.g. when the customer moves
      from one point of network attachment (POP) to another).  (See
      section A.2.3.4 "Changing Point of Network Attachment".)
   o  For each IPv6 address assignment to customers a "buffer zone"
      should be reserved that allows the customer to grow in its
      addressing range without renumbering or assignment of additional
   o  The IPv6 addressing schema of the SP must deal with multiple-
      attachments of a single customer to the SP network infrastructure
      (i.e. multi-homed network access with the same SP).

   These few requirements are only part of all the requirements a
   Service Provider has to investigate and keep in mind during the
   definition phase of its addressing architecture.  Each SP will most
   likely add more constraints to this list.

A.2.1.4.  A Few Rules of Thumb for Designing an IPv6 ISP Addressing

   As outcome of the above enumeration of requirements regarding an ISP
   IPv6 addressing plan the following design "rules of thumb" have been
   o  No "One size fits all".  Each ISP must develop its own IPv6
      address allocation schema depending on its concrete business
      needs.  It is not practicable to design one addressing plan that
      fits for all kinds of ISPs (Small / big, Routed / MPLS-based,
      access / transit, LIR / No-LIR, etc.).
   o  The levels of IPv6 address aggregation within the ISP addressing
      schema should strongly correspond to the implemented network
      structure and their number should be minimized because of
      efficiency reasons.  It is assumed that the SPs own infrastructure
      will be addressed in a fairly flat way whereas the part of the
      customer addressing architecture should contain several levels of

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   o  Keep the number of IPv6 customer routes inside your network as
      small as necessary.  A totally flat customer IPv6 addressing
      architecture without any intermediate aggregation level will lead
      to lots of customer routes inside the SP network.  A fair trade-
      off between address aggregation levels (and hence the size of the
      internal routing table of the SP) and address conservation of the
      addressing architecture has to be found.
   o  The ISP IPv6 addressing schema should provide maximal flexibility.
      This has to be realized for supporting different sizes of customer
      IPv6 address aggregates ("big" customers vs. "small" customers) as
      well as to allow future growing rates (e.g. of customer
      aggregates) and possible topological or infrastructural changes.
   o  A limited number of aggregation levels and sizes of customer
      aggregates will ease the management of the addressing schema.
      This has to be weighed against the previous "thumb rule" -

A.2.2.  Exemplary IPv6 Address Allocation Plan for a Service Provider

   In this example, the Service Provider is assumed to operate an MPLS
   based backbone and implements 6PE [RFC4798] to provide IPv6 backbone
   transport between the different locations (POPs) of a fully dual-
   stacked network access and aggregation area.

   Besides that it is assumed that the Service Provider:
   o  has received a /20 from its RIR
   o  operates its own LIR
   o  has to address its own IPv6 infrastructure
   o  delegates prefixes from this aggregate to its customers

   This addressing schema should illustrate how the /20 IPv6 prefix of
   the SP can be used to address the SP-own infrastructure and to
   delegate IPv6 prefixes to its customers following the above mentioned
   requirements and rules of thumb as far as possible.

   The below figure summarizes the device types in a SP network and the
   typical network design of a MPLS-based service provider.  The network
   hierarchy of the SP has to be taken into account for the design of an
   IPv6 addressing schema and defines its basic shape and the various
   levels of aggregation.

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   |               LSRs of the MPLS Backbone of the SP                |
      |        |             |              |                 |
      |        |             |              |                 |
   +-----+  +-----+     +--------+     +--------+         +--------+
   | LER |  | LER |     | LER-BB |     | LER-BB |         | LER-BB |
   +-----+  +-----+     +--------+     +--------+         +--------+
    |   |    |   |        |    |      /     |              |     |
    |   |    |   |        |    |     /      |              |     |
    |   |    |   |  +------+  +------+   +------+          |     |
    |   |    |   |  |BB-RAR|  |BB-RAR|   |  AG  |          |     |
    |   |    |   |  +------+  +------+   +------+          |     |
    |   |    |   |    |  |      |  |      |    |           |     |
    |   |    |   |    |  |      |  |      |    |           |     |
    |   |    |   |    |  |      |  | +-----+  +-----+  +-----+  +-----+
    |   |    |   |    |  |      |  | | RAR |  | RAR |  | RAR |  | RAR |
    |   |    |   |    |  |      |  | +-----+  +-----+  +-----+  +-----+
    |   |    |   |    |  |      |  |  |   |    |   |    |   |    |   |
    |   |    |   |    |  |      |  |  |   |    |   |    |   |    |   |
   |                       Customer networks                           |
   Figure: Exemplary Service Provider Network

   LSR    ... Label Switch Router
   LER    ... Label Edge Router
   LER-BB ... Broadband Label Edge Router
   RAR    ... Remote Access Router
   BB-RAR ... Broadband Remote Access Router
   AG     ... Aggregation Router

   Basic design decisions for the exemplary Service Provider IPv6
   address plan regarding customer prefixes take into consideration:
   o  The prefixes assigned to all customers behind the same LER (e.g.
      LER or LER-BB) are aggregated under one LER prefix.  This ensures
      that the number of labels that have to be used for 6PE is limited
      and hence provides a strong MPLS label conservation.
   o  The /20 prefix of the SP is separated into 3 different pools that
      are used to allocate IPv6 prefixes to the customers of the SP:
      *  A pool (e.g. /24) for satisfying the addressing needs of really
         "big" customers (as defined in A.2.2.1 sub-section A.) that
         need IPv6 prefixes larger than /48 (e.g. /32).  These customers
         are assumed to be connected to several POPs of the access
         network, so that this customer prefix will be visible in each
         of these POPs.

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      *  A pool (e.g. /24) for the LERs with direct customer connections
         (e.g. dedicated line access) and without an additional
         aggregation area between the customer and the LER.  (These LERs
         are mostly connected to a limited number of customers because
         of the limited number of interfaces/ports.)
      *  A larger pool (e.g. 14*/24) for LERs (e.g.  LER-BB) that serve
         a high number of customers that are normally connected via some
         kind of aggregation network (e.g.  DSL customers behind a BB-
         RAR or Dial-In customers behind a RAR).
      *  The IPv6 address delegation within each Pool (end customer
         delegation or also the aggregates that are dedicated to the
         LERs itself) should be chosen with an additional buffer zone of
         100% - 300% for future growth.  I.e. 1 or 2 additional prefix
         bits should be reserved according to the expected future growth
         rate of the corresponding customer / the corresponding network
         device aggregate.

A.2.2.1.  Defining an IPv6 Address Allocation Plan for Customers of the
          Service Provider

A.  'Big' Customers

   SP's "big" customers receive their prefix from the /24 IPv6 address
   aggregate that has been reserved for their "big" customers.  A
   customer is considered as "big" customer if it has a very complex
   network infrastructure and/or huge IPv6 address needs (e.g. because
   of very large customer numbers) and/or several uplinks to different
   POPs of the SP network.

   The assigned IPv6 address prefixes can have a prefix length in the
   range 32-48 and for each assignment a 100 or 300% future growing zone
   is marked as "reserved" for this customer.  This means for instance
   that with a delegation of a /34 to a customer the corresponding /32
   prefix (which contains this /34) is reserved for the customers future

   The prefixes for the "big" customers can be chosen from the
   corresponding "big customer" pool by either using an equidistant
   algorithm or using mechanisms similar to the Sparse Allocation
   Algorithm (SAA) [reference2].

A.  'Common' Customers

   All customers that are not "big" customers are considered as "common"
   customers.  They represent the majority of customers hence they
   receive a /48 out of the IPv6 customer address pool of the LER where
   they are directly connected or aggregated.

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   Again a 100 - 300% future growing IPv6 address range is reserved for
   each customer, so that a "common" customer receives a /48 allocation
   but has a /47 or /46 reserved.

   (Note: If it is obvious that the likelyhood of needing a /47 or /46
   in the future is very small for a "common" customer, than no growing
   buffer should be reserved for it and only a /48 will be assigned
   without any growing buffer.)

   In the network access scenarios where the customer is directly
   connected to the LER the customer prefix is directly taken out of the
   customer IPv6 address aggregate (e.g. /38) of the corresponding LER.

   In all other cases (e.g. the customer is attached to a RAR that is
   themselves aggregated to an AG or to a LER-BB) at least 2 different
   approaches are possible.

   1) Mapping of Aggregation Network Hierarchy into Customer IPv6
   Addressing Schema.  The aggregation network hierarchy could be mapped
   into the design of the customer prefix pools of each network level in
   order to achieve a maximal aggregation at the LER level as well as at
   the intermediate levels.  (Example: Customer - /48, RAR - /38, AG -
   /32, LER-BB - /30).  At each network level an adequate growing zone
   should be reserved.  (Note: This approach requires of course some
   "fine tuning" of the addressing schema based on a very good knowledge
   of the Service Provider network topology including actual growing
   ranges and rates.)

   When the IPv6 customer address pool of a LER (or another device of
   the aggregation network - AG or RAR) is exhausted, the related LER
   (or AG or RAR) prefix is shortened by 1 or 2 bits (e.g. from /38 to
   /37 or /36) so that the originally reserved growing zone can be used
   for further IPv6 address allocations to customers.  In the case where
   this growing zone is exhausted as well a new prefix range from the
   corresponding pool of the next higher hierarchy level can be

   2) "Flat" Customer IPv6 Addressing Schema.  The other option is to
   allocate all the customer prefixes directly out of the customer IPv6
   address pool of the LER where the customers are attached and
   aggregated and to ignore the intermediate aggregation network
   infrastructure.  This approach leads of course to a higher amount of
   customer routes at LER and aggregation network level but takes a
   great amount of complexity out of the addressing schema.
   Nevertheless the aggregation of the customer prefixes to one prefix
   at LER level is realized as required above.

   (Note: The handling of (e.g. technically triggered) changes within

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   the ISP access network is shortly discussed in section A.2.3.5.)

   If the actual observed growing rates show that the reserved growing
   zones are not needed than these growing areas can be freed and used
   for assignments for prefix pools to other devices at the same level
   of the network hierarchy.

A.2.2.2.  Defining an IPv6 Address Allocation Plan for the Service
          Provider Network Infrastructure

   For the IPv6 addressing of SPs own network infrastructure a /32 (or
   /40) from the "big" customers address pool can be chosen.

   This SP infrastructure prefix is used to code the network
   infrastructure of the SP by assigning a /48 to every POP/location and
   using for instance a /56 for coding the corresponding router within
   this POP.  Each SP internal link behind a router interface could be
   coded using a /64 prefix.  (Note: While it is suggested to choose a
   /48 for addressing the POP/location of the SP network it is left to
   each SP to decide what prefix length to assign to the routers and
   links within this POP.)

   The IIDs of the router interfaces may be generated by using EUI-64 or
   through plain manual configuration e.g. for coding additional network
   or operational information into the IID.

   It is assumed that again 100 - 300% growing zones for each level of
   network hierarchy and additional prefix bits may be assigned to POPs
   and/or routers if needed.

   Loopback interfaces of routers may be chosen from the first /64 of
   the /56 router prefix (in the example above).

   (Note: The /32 (or /40) prefix that has been chosen for addressing
   SPs own IPv6 network infrastructure gives enough place to code
   additional functionalities like security levels or private and test
   infrastructure although such approaches haven't been considered in
   more detail for the above described SP until now.)

   Point-to-point links to customers (e.g.  PPP links, dedicated line
   etc.) may be addressed using /126 prefixes out of the first /64 of
   the access routers that could be reserved for this reason.

A.2.3.  Additional Remarks

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A.2.3.1.  ULA

   From the actual view point of SP there is no compelling reason why
   ULAs should be used from a SP.  Look at section 2.2.

   ULAs could be used inside the SP network in order to have an
   additional "site-local scoped" IPv6 address for SPs own
   infrastructure for instance for network management reasons and maybe
   also in order to have an addressing schema that couldn't be reached
   from outside the SP network.

   In the case when ULAs are used it is possible to map the proposed
   internal IPv6 addressing of SPs own network infrastructure as
   described in A.2.2.2 above directly to the ULA addressing schema by
   substituting the /48 POP prefix with a /48 ULA site prefix.

A.2.3.2.  Multicast

   IPv6 Multicast-related addressing issues are out of the scope of this

A.2.3.3.  POP Multi-homing

   POP (or better LER) Multi-homing of customers with the same SP can be
   realized within the proposed IPv6 addressing schema of the SP by
   assigning multiple LER-dependent prefixes to this customer (i.e.
   considering each customer location as a single-standing customer) or
   by choosing a customer prefix out of the pool of "big" customers.
   The second solution has the disadvantage that in every LER where the
   customer is attached this prefix will appear inside the IGP routing
   table requiring an explicit MPLS label.

   (Note: The described negative POP/LER Multi-homing effects to the
   addressing architecture in the SP access network are not tackled by
   implementing the Shim6 Site Multi-homing approach since this approach
   targets only on a mechanism for dealing with multiple prefixes in end
   systems -- the SP will nevertheless have unaggregated customer
   prefixes in its internal routing tables.)

A.2.3.4.  Changing Point of Network Attachement

   In the possible case that a customer has to change its point of
   network attachment to another POP/LER within the ISP access network
   two different approaches can be applied assuming that the customer
   uses PA addresses out of the SP aggregate:

   1.)  The customer has to renumber its network with an adequate
   customer prefix out of the aggregate of the corresponding LER/RAR of

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   its new network attachement.  To minimise the administrative burden
   for the customer the prefix should be of the same size as the former.
   This conserves the IPv6 address aggregation within the SP network
   (and the MPLS label space) but adds additional burden to the
   customer.  Hence this approach will most likely only be chosen in the
   case of "small customers" with temporary addressing needs and/or
   prefix delegation with address auto-configuration.

   2.)  The customer does not need to renumber its network and keeps its
   address aggregate.

   This apporach leads to additional more-specific routing entries
   within the IGP routing table of the LER and will hence consume
   additional MPLS labels - but it is totally transparent to the
   customer.  Because this results in additional administrative effort
   and will stress the router resources (label space, memory) of the ISP
   this solution will only be offered to the most valuable customers of
   an ISP (like e.g. "big customers" or "enterprise customers").

   Nevertheless the ISP has again to find a fair trade-off between
   customer renumbering and sub-optimal address aggregation (i.e. the
   generation of additional more-specific routing entries within the IGP
   and the waste of MPLS Label space).

A.2.3.5.  Restructuring of SP (access) Network and Renumbering

   A technically triggered restructuring of the SP (access) network (for
   instance because of split of equipment or installation of new
   equipment) should not lead to a customer network renumbering.  This
   challenge should be handled in advance by an intelligent network
   design and IPv6 address planing.

   In the worst case the customer network renumbering could be avoided
   through the implementation of more specific customer routes.  (Note:
   Since this kind of network restructuring will mostly happen within
   the access network (at the level) below the LER, the LER aggregation
   level will not be harmed and the more-specific routes will not
   consume additional MPLS label space.)

A.2.3.6.  Extensions Needed for the Later IPv6 Migration Phases

   The proposed IPv6 addressing schema for a SP needs some slight
   enhancements / modifications for the later phases of IPv6
   integration, for instance in the case when the whole MPLS backbone
   infrastructure (LDP, IGP etc.) is realized over IPv6 transport and an
   IPv6 addressing of the LSRs is needed.  Other changes may be
   necessary as well but should not be explained at this point.

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Appendix B.  Considerations for Subnet Prefixes Different then /64

B.1.  Considerations for Subnet Prefixes Shorter then /64

   An allocation of a prefix shorter then 64 bits to a node or interface
   is considered bad practice.  One exception to this statement is when
   using 6to4 technology where a /16 prefix is utilized for the pseudo-
   interface [RFC3056].  The shortest subnet prefix that could
   theoretically be assigned to an interface or node is limited by the
   size of the network prefix allocated to the organization.

   A possible reason for choosing the subnet prefix for an interface
   shorter then /64 is that it would allow more nodes to be attached to
   that interface compared to a prescribed length of 64 bits.  This
   however is unnecessary for most networks considering that 2^64
   provides plenty of node addresses.

   The subnet prefix assignments can be made either by manual
   configuration, by a stateful Host Configuration Protocol [RFC3315],
   by a stateful prefix delegation mechanism [RFC3633] or implied by
   stateless autoconfiguration from prefix RAs.

B.2.  Considerations for Subnet Prefixes Longer then /64

   The following subsections describe subnet prefix values that should
   be avoided in deployments, because nodes who assume that the subnet
   prefix is /64 could treat them incorrectly.

B.2.1.  /126 Addresses

   126 bit subnet prefixes are typically used for point-to-point links
   similar to a the IPv4 address conservative /30 allocation for point-
   to-point links.  The usage of this subnet address length does not
   lead to any additional considerations other than the ones discussed
   earlier in this section, particularly those related to the "u" and
   "g" bits.

B.2.2.  /127 Addresses

   The usage of the /127 addresses, the equivalent of IPv4's RFC3021
   [RFC3021] is not valid and should be strongly discouraged as
   documented in RFC3627 [RFC3627].

B.2.3.  /128 Addresses

   The 128 bit address prefix may be used in those situations where we
   know that one, and only one address is sufficient.  Example usage
   would be the off-link loopback address of a network device.

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   When choosing a 128 bit prefix, it is recommended to take the "u" and
   "g" bits into consideration and to make sure that there is no overlap
   with either the following well-known addresses:
   o  Subnet Router Anycast Address
   o  Reserved Subnet Anycast Address
   o  Addresses used by Embedded-RP
   o  ISATAP Addresses

B.2.4.  EUI-64 'u' and 'g' bits

   When using subnet prefix lengths other than /64, the interface
   identifier cannot be in Modified EUI-64 format as required by
   [RFC4291].  However, nodes not aware that a prefix length other than
   /64 is used might still think it's an EUI-64; therefore, it's prudent
   to take the next considerations to set the bits into account.

   Address space conservation is the main motivation for using a subnet
   prefix length longer than 64 bits, however this kind of address
   conservation is of little benefit compared with the additional
   considerations one must make when creating and maintain an IPv6
   address plan.

   The address assignment can be made either by manual configuration or
   by a stateful Host Configuration Protocol [RFC3315].

   When assigning a subnet prefix of more then 70 bits, according to
   RFC4291 [RFC4291] 'u' and 'g' bits (respectively the 71st and 72nd
   bit) need to be taken into consideration and should be set correct.

   The 'u' (universal/local) bit is the 71st bit of IPv6 address and is
   used to determine whether the address is universally or locally
   administered.  If 0, the IEEE, through the designation of a unique
   company ID, has administered the address.  If 1, the address is
   locally administered.  The network administrator has overridden the
   manufactured address and specified a different address.

   The 'g' (the individual/group) bit is the 72st bit and is used to
   determine whether the address is an individual address (unicast) or a
   group address (multicast).  If '0', the address is a unicast address.
   If '1', the address is a multicast address.

   In current IPv6 protocol stacks, the relevance of the 'u' and 'g' bit
   is marginal and typically will not show an issue when configured
   wrongly, however future implementations may turn out differently if
   they would be processing the 'u' and 'g' bit in IEEE like behavior.

   When using subnet lengths longer then 64 bits, it is important to
   avoid selecting addresses that may have a predefined use and could

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   confuse IPv6 protocol stacks.  The alternate usage may not be a
   simple unicast address in all cases.  The following points should be
   considered when selecting a subnet length longer then 64 bits.

B.2.5.  Anycast Addresses

B.2.5.1.  Subnet Router Anycast Address

   RFC4291 [RFC4291] provides a definition for the required Subnet
   Router Anycast Address as follows:

    |                   n bits                   |   128-n bits   |
    |               subnet prefix                | 00000000000000 |

   It is recommended to avoid allocating this IPv6 address to a device
   which expects to have a normal unicast address.  There is no
   additional dependency for the subnet prefix with the exception of the
   64-bit extended unique identifier (EUI-64) and an Interface
   Identifier (IID) dependency.  These will be discussed later in this

B.2.5.2.  Reserved IPv6 Subnet Anycast Addresses

   RFC2526 [RFC2526] stated that within each subnet, the highest 128
   interface identifier values are reserved for assignment as subnet
   anycast addresses.

   The construction of a reserved subnet anycast address depends on the
   type of IPv6 addresses used within the subnet, as indicated by the
   format prefix in the addresses.

   The first type of Subnet Anycast addresses have been defined as
   follows for EUI-64 format:

    |           64 bits            |      57 bits     |   7 bits   |
    |        subnet prefix         | 1111110111...111 | anycast ID |

   The anycast address structure implies that it is important to avoid

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   creating a subnet prefix where the bits 65 to 121 are defined as
   "1111110111...111" (57 bits in total) so that confusion can be

   For other IPv6 address types (that is, with format prefixes other
   than those listed above), the interface identifier is not in 64-bit
   extended unique identifier (EUI-64) format and may be other than 64
   bits in length; these reserved subnet anycast addresses for such
   address types are constructed as follows:

    |           n bits             |    121-n bits    |   7 bits   |
    |        subnet prefix         | 1111111...111111 | anycast ID |
                                   |   interface identifier field  |

   It is recommended to avoid allocating this IPv6 address to a device
   which expects to have a normal unicast address.  There is no
   additional dependency for the subnet prefix with the exception of the
   EUI-64 and an Interface Identifier (IID) dependency.  These will be
   discussed later in this document.

B.2.6.  Addresses Used by Embedded-RP (RFC3956)

   Embedded-RP [RFC3956] reflects the concept of integrating the
   Rendezvous Point (RP) IPv6 address into the IPv6 multicast group
   address.  Due to this embedding and the fact that the length of the
   IPv6 address AND the IPv6 multicast address are 128 bits, it is not
   possible to have the complete IPv6 address of the multicast RP
   embedded as such.

   This resulted in a restriction of 15 possible RP-addresses per prefix
   that can be used with embedded-RP.  The space assigned for the
   embedded-RP is based on the 4 low order bits, while the remainder of
   the Interface ID (RIID) is set to all '0'.

               (IPv6-prefix (64 bits))(60 bits all '0')(RIID)

                   Where: (RIID) = 4 bit.

   This format implies that when selecting subnet prefixes longer then
   64, and the bits beyond the 64th one are non-zero, the subnet can not
   use embedded-RP.

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   In addition it is discouraged to assign a matching embedded-RP IPv6
   address to a device that is not a real Multicast Rendezvous Point,
   even though it would not generate major problems.

B.2.7.  ISATAP Addresses

   ISATAP [RFC5214] is an experimental automatic tunneling protocol used
   to provide IPv6 connectivity over an IPv4 campus or enterprise
   environment.  In order to leverage the underlying IPv4
   infrastructure, the IPv6 addresses are constructed in a special

   An IPv6 ISATAP address has the IPv4 address embedded, based on a
   predefined structure policy that identifies them as an ISATAP

                [IPv6 Prefix (64 bits)][0000:5EFE][IPv4 address]

   When using subnet prefix length longer then 64 bits it is good
   engineering practice that the portion of the IPv6 prefix from bit 65
   to the end of the host-id does not match with the well-known ISATAP
   [0000:5EFE] address when assigning an IPv6 address to a non-ISATAP

   Note that the definition of ISATAP does not support multicast.

Authors' Addresses

   Gunter Van de Velde
   Cisco Systems
   De Kleetlaan 6a
   Diegem  1831

   Phone: +32 2704 5473

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   Ciprian Popoviciu
   Cisco Systems
   7025-6 Kit Creek Road
   Research Triangle Park, North Carolina  PO Box 14987

   Phone: +1 919 392-3723

   Tim Chown
   University of Southampton
   Southampton,   SO17 1BJ
   United Kingdom

   Phone: +44 23 8059 3257

   Olaf Bonness
   T-Systems Enterprise Services GmbH
   Goslarer Ufer 35
   Berlin,   10589

   Phone: +49 30 3497 3124

   Christian Hahn
   T-Systems Enterprise Services GmbH
   Goslarer Ufer 35
   Berlin,   10589

   Phone: +49 30 3497 3164

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Full Copyright Statement

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