INTERNET-DRAFT                                          R. Hinden, Nokia
September 23, 2003                               Brian Haberman, Caspian

                  Unique Local IPv6 Unicast Addresses


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   To view the list Internet-Draft Shadow Directories, see

   This internet draft expires on March 28, 2004.


   This document defines an unicast address format that is globally
   unique and is intended for local communications, usually inside of a
   site.  They are not expected to be routable on the global Internet
   given current routing technology.

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1.0 Introduction

   This document defines an IPv6 unicast address format that is globally
   unique and is intended for local communications [IPV6].  These
   addresses are called Unique Local IPv6 Unicast Addresses and are
   abbreviated in this document as Local IPv6 addresses.  They are not
   expected to be routable on the global Internet given current routing
   technology.  They are routable inside of a more limited area such as
   a site.  They may also be routed between a limited set of sites.

   Local IPv6 unicast addresses have the following characteristics:

      - Globally unique prefix.
      - Well known prefix to allow for easy filtering at site
      - Allows sites to be combined or privately interconnected without
        creating any address conflicts or require renumbering of
        interfaces using these prefixes.
      - Internet Service Provider independent and can be used for
        communications inside of a site without having any permanent or
        intermittent Internet connectivity.
      - If accidentally leaked outside of a site via routing or DNS,
        there is no conflict with any other addresses.
      - In practice, applications may treat these address like global
        scoped addresses.

   This document defines the format of Local IPv6 addresses, how to
   allocate them, and usage considerations including routing, site
   border routers, DNS, application support, VPN usage, and guidelines
   for how to use for local communication inside a site.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC 2119].

2.0 Acknowledgments

   The underlying idea of creating Local IPv6 addresses described in
   this document been proposed a number of times by a variety of people.
   The authors of this draft do not claim exclusive credit.  Credit goes
   to Brian Carpenter, Christian Huitema, Aidan Williams, Andrew White,
   Charlie Perkins, and many others.  The authors would also like to
   thank Brian Carpenter, Charlie Perkins, Harald Alvestrand, Keith
   Moore, Margaret Wasserman, Shannon Behrens, Alan Beard, Hans Kruse,
   and Geoff Huston for their comments and suggestions on this document.

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3.0 Local IPv6 Unicast Addresses

3.1 Format

   The Local IPv6 addresses are created using a centrally allocated
   global ID.  They have the following format:

      |   n    |
      |  bits  |  m bits    |  16 bits  |          64 bits            |
      | prefix | global ID  | subnet ID |        interface ID         |


      prefix            prefix to identify Local IPv6 unicast addresses.

      global ID         global identifier used to create a globally
                        unique prefix. See section 3.2 for additional

      subnet ID         16-bit subnet ID is an identifier of a subnet
                        within the site.

      interface ID      64-bit IID as defined in [ADDARCH].

   There are a range of choices available when choosing the size of the
   prefix and Global ID field length.  There is a direct tradeoff
   between having a Global ID field large enough to support foreseeable
   future growth and not using too much of the IPv6 address space
   needlessly.  A reasonable way of evaluating a specific field length
   is to compare it to a projected 2050 world population of 9.3 billion
   [POPUL] to compare the number of resulting /48 prefixes per person.
   A range of prefix choices is shown in the following table:

      Prefix   Global ID      Number /48          Prefixes     % of IPv6
               Length         Prefixes            per Person   Address Space

      /11      37               137,438,953,472      15        0.049%
      /10      38               274,877,906,944      30        0.098%
      /9       39               549,755,813,888      59        0.195%
      /8       40             1,099,511,627,776     118        0.391%
      /7       41             2,199,023,255,552     236        0.781%
      /6       42             4,398,046,511,104     473        1.563%

   A very high utilization ratio of these allocations can be assumed
   because no internal structure is required in the field nor is there
   any reason to be able to aggregate the prefixes.

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   The authors believes that a /7 prefix resulting in a 41 bit Global ID
   is a good choice.  It provides for a large number of assignments
   (i.e., 2.2 trillion) and at the same time uses less than .8% of the
   total IPv6 address space.  It is unlikely that this space will be
   exhausted.  If more than this was needed, then additional IPv6
   address space could be allocated for this purpose.

   For the rest of this document the FC00::/7 prefix and a 41-bit Global
   ID is used.

3.2 Global ID

   The allocation of global IDs should be pseudo-random [RANDOM].  They
   should not be assigned sequentially or with well known numbers.  This
   to ensure that there is not any relationship between allocations and
   to help clarify that these prefixes are not intended to be routed
   globally.  Specifically, these prefixes are designed to not

   There are two ways to allocate Global IDs.  These are centrally by a
   allocation authority and locally by the site.  The Global ID is split
   into two parts for each type of allocation.  The prefixes for each
   type are:

      FC00::/8    Centrally assigned
      FD00::/8    Locally assigned

   Each results in a 40-bit space to allocate.

   Two assignment methods are included because they have different
   properties.  The centrally assigned global IDs have a much higher
   probability that they are unique and the assignments can be escrowed
   to resolve any disputes regarding duplicate assignments.  The local
   assignments are free and do not need any central coordination or
   assignment, but have a lower (but still adequate) probability of
   being unique.  It is expected that large managed sites will prefer
   central assignments and small or disconnected sites will prefer local
   assignments.  It is recommended that sites planning to use Local IPv6
   addresses for extensive inter-site communication use a centrally
   assigned prefix as the possibility of any conflicts is lower.  Sites
   are free to choose either approach.

3.2.1 Centrally Assigned Global IDs

   Centrally assigned global IDs MUST be generated with a pseudo-random
   algorithm consistent with [RANDOM].  They should not be assigned

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   sequentially or by locality.  This to ensure that there is not any
   relationship between allocations and to help clarify that these
   prefixes are not intended to be routed globally.  Specifically, these
   prefixes are designed to not aggregate.

   Global IDs should be assigned centrally by a single allocation
   authority because they are pseudo-random and without any structure.
   This is easiest to accomplish if there is a single source of the

   The requirements for centrally assigned global ID allocations are:

      - Available to anyone in an unbiased manner.
      - Permanent with no periodic fees.
      - One time non-refundable allocation fee in the order of 10 Euros
        (at January 1, 2004 exchange rates) per allocation.
      - The ownership of each individual allocation should be private,
        but should be escrowed.

   The allocation authority should permit allocations to be obtained
   without having any sort of internet connectivity.  For example in
   addition to web based registration they should support some methods
   like telephone, postal mail, fax, telex, etc.  They should also
   accept a variety of payment methods and currencies.

   The reason for the one time 10 Euro charge for each prefix is to
   provide a barrier to any hoarding of the these allocations but at the
   same time keep the cost low enough to not create a barrier to anyone
   needing one.  The charge is one time to eliminate the need for an
   ongoing relationship with the allocation authority.  The charge is
   non-refundable in order to keep overhead low.

   The ownership of the allocations is not needed to be public since the
   resulting addresses are intended to be used for local communication.
   It is escrowed to insure there are no duplicate allocations and in
   case it is needed in the future (e.g., to resolve duplicate
   allocation disputes).

   Note, there are many possible ways of of creating an allocation
   authority.  It is important to keep in mind when reviewing
   alternatives that the goal is to pick one that can do the job.  It
   doesn't have to be perfect, only good enough to do the job at hand.

   This document directs the IANA, in section 13.0, to delegate the
   FC00::/8 prefix to an allocation authority to allocate centrally
   assigned /48 prefixes consistent with the requirements defined in
   this section.

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3.2.2 Locally Assigned Global IDs

   Global IDs can also be generated locally by an individual site.  This
   makes it easy to get a prefix with out the need to contact an
   assignment authority or internet service provider.  There is not as
   high a degree of assurance that the prefix will not conflict with
   another locally generated prefix, but the likelihood of conflict is
   small.  Sites that are not comfortable with this degree of
   uncertainty should use a centrally assigned global ID.

   Locally assigned global IDs MUST be generated with a pseudo-random
   algorithm consistent with [RANDOM].  Section 3.2.3 describes a
   suggested algorithm.  It is important to insure a reasonable
   likelihood uniqueness that all sites generating a Global IDs use a
   functionally similar algorithm.

3.2.3  Sample Code for Pseudo-Random Global ID Algorithm

   The algorithm described below is intended to be used for centrally
   and locally assigned Global IDs.  In each case the resulting global
   ID will be used in the appropriate prefix as defined in section 3.2.

     1) Obtain the current time of day in 64-bit NTP format [NTP].
     2) Obtain an EUI-64 identifier from the system running this
        algorithm.  If an EUI-64 does not exist, one can be created from
        a 48-bit MAC address as specified in [ADDARCH].  If an EUI-64
        cannot be obtained or created, a suitably unique identifier,
        local to the node, should be used (e.g. system serial number).
     3) Concatenate the time of day with the system-specific identifier
        creating a key.
     4) Compute an MD5 digest on the key as specified in [MD5DIG].
     5) Use the least significant 40 bits as the Global ID.

   This algorithm will result in a global ID that is reasonably unique
   and can be used as a Global ID.

3.2.4  Analysis of the Uniqueness of Global IDs

   The selection of a pseudo random global ID is similar to the
   selection of an SSRC identifier in RTP/RTCP defined in section 8.1 of
   [RTP].  This analysis is adapted from that document.

   Since the global ID is chosen randomly, it is possible that two or
   more networks that have an inter-network connection using globally-
   unique local addresses will chose the same global ID.  The
   probability of collision can be approximated based on the number of

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   inter-connections using globally-unique local addresses and the
   length of the ID (40 bits).  The formula

      P = 1 - exp(-N**2 / 2**(L+1))

   approximates the probability of collision (where N is the number
   inter-connections and L is the length of the global ID).

   The following table shows the probability of a collision for a range
   of inter-connections using a 40 bit global ID field.

      Inter-connections    Probability of Collision

          2                1.81**-12
         10                4.54**-11
        100                4.54**-09
       1000                4.54**-07
      10000                4.54**-05

   Based on this analysis the uniqueness of locally generated global IDs
   is adequate for sites planning a small to moderate amount inter-site
   communication using locally generated global IDs.  Sites planning
   more extensive inter-site communication should consider using the
   centrally assigned global ID.

3.3 Scope Definition

   By default, the scope of these addresses is global.  That is, they
   are not limited by ambiguity like the site-local addresses defined in
   [ADDARCH].  Rather, these prefixes are globally unique, and as such,
   their applicability exceeds the current site-local addresses.  Their
   limitation is in the routability of the prefixes, which is limited to
   a site and any explicit routing agreements with other sites to
   propagate them.  Also, unlike site-locals, these prefixes can overlap
   each other.

4.0 Routing

   Local IPv6 addresses are designed to be routed inside of a site in
   the same manner as other types of unicast addresses.  They can be
   carried in any IPv6 routing protocol without any change.

   It is expected that they would share the same subnet IDs with
   provider based global unicast addresses if they were being used
   concurrently [GLOBAL].

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   Any routing protocol that is used between sites MUST filter out any
   incoming or outgoing Local IPv6 unicast routes.  The exception to
   this is if specific /48 IPv6 local unicast routes have been
   configured to allow for inter-site communication.

   If BGP is being used at the site border with an ISP, filters MUST be
   installed by default in the BGP configuration to keep any Local IPv6
   address prefixes from being advertised outside of the site or for
   these prefixes to be learned from another site.  The exception to
   this is if there are specific /48 routes created for one or more
   Local IPv6 prefixes.

5.0 Renumbering and Site Merging

   The use of Local IPv6 addresses in a site results in making
   communication using these addresses independent of renumbering a
   site's provider based global addresses.

   When merging multiple sites none of the addresses created with these
   prefixes need to be renumbered because all of the addresses are
   unique.  Routes for each specific prefix would have to be configured
   to allow routing to work correctly between the formerly separate

6.0 Site Border Router and Firewall Packet Filtering

   While no serious harm will be done if packets with these addresses
   are sent outside of a site via a default route, it is recommended
   that they be filtered to keep any packets with Local IPv6 destination
   addresses from leaking outside of the site and to keep any site
   prefixes from being advertised outside of their site.

   Site border routers SHOULD install a black hole route for the Local
   IPv6 prefix FC00::/7.  This will insure that packets with Local IPv6
   destination addresses will not be forwarded outside of the site via a
   default route.

   Site border routers and firewalls SHOULD NOT forward any packets with
   Local IPv6 source or destination addresses outside of the site unless
   they have been explicitly configured with routing information about
   other Local IPv6 prefixes.  The default behavior of these devices
   SHOULD be to filter them.

   Additionally, domain border routers connecting autonomous systems
   throughout the Internet SHOULD obey these recommendations for site
   border routers.

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7.0 DNS Issues

   AAAA records for Local IPv6 addresses SHOULD NOT be installed in the
   global DNS.  They may be installed in a naming system local to the
   site or kept separate from the global DNS using techniques such as
   "two-faced" DNS.

   If Local IPv6 address are configured in the global DNS, no harm is
   done because they are unique and will not create any confusion.  They
   may not be reachable, but this is a property that is common to all
   types of global IPv6 unicast addresses.

   For future study names with Local IPv6 addresses may be resolved
   inside of the site using dynamic naming systems such as Multicast

8.0 Application and Higher Level Protocol Issues

   Application and other higher level protocols can treat Local IPv6
   addresses in the same manner as other types of global unicast
   addresses.  No special handling is required.  This type of addresses
   may not be reachable, but that is no different from other types of
   IPv6 global unicast addresses.  Applications need to be able to
   handle multiple addresses that may or may not be reachable any point
   in time.  In most cases this complexity should be hidden in APIs.

   From a host's perspective this difference shows up as different
   reachability than global unicast and could be handled by default that
   way.  In some cases it is better for nodes and applications to treat
   them differently from global unicast addresses.  A starting point
   might be to give them preference over global unicast, but fall back
   to global unicast if a particular destination is found to be
   unreachable.  Much of this behavior can be controlled by how they are
   allocated to nodes and put into the DNS.  However it is useful if a
   host can have both types of addresses and use them appropriately.

   Note that the address selection mechanisms of [ADDSEL], and in
   particular the policy override mechanism replacing default address
   selection, are expected to be used on a site where Local IPv6
   addresses are configured.

9.0 Use of Local IPv6 Addresses for Local Communications

   Local IPv6 addresses, like global scope unicast addresses, are only
   assigned to nodes if their use has been enabled (via IPv6 address
   autoconfiguration [ADDAUTO], DHCPv6 [DHCP6], or manually) and

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   configured in the DNS.  They are not created automatically the way
   that IPv6 link-local addresses are and will not appear or be used
   unless they are purposely configured.

   In order for hosts to autoconfigure Local IPv6 addresses, routers
   have to be configured to advertise Local IPv6 /64 prefixes in router
   advertisements.  Likewise, a DHCPv6 server must have been configured
   to assign them.  In order for a node to learn the Local IPv6 address
   of another node, the Local IPv6 address must have been installed in
   the DNS.  For these reasons, it is straight forward to control their
   usage in a site.

   To limit the use of Local IPv6 addresses the following guidelines

      - Nodes that are to only be reachable inside of a site:  The local
        DNS should be configured to only include the Local IPv6
        addresses of these nodes.  Nodes with only Local IPv6 addresses
        must not be installed in the global DNS.

      - Nodes that are to be limited to only communicate with other
        nodes in the site:  These nodes should be set to only
        autoconfigure Local IPv6 addresses via [ADDAUTO] or to only
        receive Local IPv6 addresses via [DHCP6].  Note: For the case
        where both global and Local IPv6 prefixes are being advertised
        on a subnet, this will require a switch in the devices to only
        autoconfigure Local IPv6 addresses.

      - Nodes that are to be reachable from inside of the site and from
        outside of the site:  The DNS should be configured to include
        the global addresses of these nodes.  The local DNS may be
        configured to also include the Local IPv6 addresses of these

      - Nodes that can communicate with other nodes inside of the site
        and outside of the site: These nodes should autoconfigure global
        addresses via [ADDAUTO] or receive global address via [DHCP6].
        They may also obtain Local IPv6 addresses via the same

10.0 Use of Local IPv6 Addresses with VPNs

   Local IPv6 addresses can be used for inter-site Virtual Private
   Networks (VPN) if appropriate routes are set up.  Because the
   addresses are unique these VPNs will work reliably and without the
   need for translation.  They have the additional property that they
   will continue to work if the individual sites are renumbered or

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11.0 Advantages and Disadvantages

11.1 Advantages

   This approach has the following advantages:

      - Provides Local IPv6 prefixes that can be used independently of
        any provider based IPv6 unicast address allocations.  This is
        useful for sites not always connected to the Internet or sites
        that wish to have a distinct prefix that can be used to localize
        traffic inside of the site.
      - Applications can treat these addresses in an identical manner as
        any other type of global IPv6 unicast addresses.
      - Sites can be merged without any renumbering of the Local IPv6
      - Sites can change their provider based IPv6 unicast address
        without disrupting any communication using Local IPv6 addresses.
      - Well known prefix that allows for easy filtering at site
      - Can be used for inter-site VPNs.
      - If accidently leaked outside of a site via routing or DNS, there
        is no conflict with any other addresses.

11.2 Disadvantages

   This approach has the following disadvantages:

      - Not possible to route Local IPv6 prefixes on the global Internet
        with current routing technology.  Consequentially, it is
        necessary to have the default behavior of site border routers to
        filter these addresses.
      - There is a very low probability of non-unique locally assigned
        global IDs being generated by the algorithm in section 3.2.3.
        This risk can be ignored for all practical purposes, but it
        leads to a theoretical risk of clashing address prefixes.

12.0 Security Considerations

   Local IPv6 addresses do not provide any inherent security to the
   nodes that use them.  They may be used with filters at site
   boundaries to keep Local IPv6 traffic inside of the site, but this is
   no more or less secure than filtering any other type of global IPv6
   unicast addresses.

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   Local IPv6 addresses do allow for address-based security mechanisms,
   including IPSEC, across end to end VPN connections.

13.0 IANA Considerations

   The IANA is instructed to allocate the FC00::/7 prefix for Unique
   Local IPv6 unicast addresses.

   The IANA is instructed to delegate, within a reasonable time, the
   prefix FC00::/8 to an allocation authority for Unique Local IPv6
   Unicast prefixes of length /48.  This allocation authority shall
   comply with the requirements described in section 3.2 of this
   document, including in particular the charging of a modest one-time
   fee, with any profit being used for the public good in connection
   with the Internet.

14.0 References

14.1 Normative References

   [ADDARCH] Hinden, R., S. Deering, S., "IP Version 6 Addressing
             Architecture", RFC 3515, April 2003.

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

   [IPV6]    Deering, S., R. Hinden, "Internet Protocol, Version 6
             (IPv6) Specification", RFC 2460, December 1998.

   [MD5DIG]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
             April 1992.

   [NTP]     Mills, David L., "Network Time Protocol (Version 3)
             Specification, Implementation and Analysis", RFC 1305,
             March 1992.

   [POPUL]   Population Reference Bureau, "World Population Data Sheet
             of the Population Reference Bureau 2002",  August 2002.

   [RANDOM]  Eastlake, D. 3rd, S. Crocker, J. Schiller, "Randomness
             Recommendations for Security", RFC 1750, December 1994.

   [RFC2026] Bradner, S., "The Internet Standards Process -- Revision
             3", RFC 2026, BCP00009, October 1996.

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   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", RFC 2119, BCP14, March 1997.

14.2 Informative References

   [ADDAUTO] Thomson, S., T. Narten, "IPv6 Stateless Address
             Autoconfiguration", RFC 2462, December 1998.

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

   [DHCP6]   Droms, R., et. al., "Dynamic Host Configuration Protocol
             for IPv6 (DHCPv6)", RFC3315, July 2003.

   [RTP]     Schulzrinne, H., S. Casner, R. Frederick, V. Jacobson,
             "RTP: A Transport Protocol for Real-Time Applications"
             RFC1889, January 1996.

15.0 Authors' Addresses

   Robert M. Hinden
   313 Fairchild Drive
   Mountain View, CA 94043

   phone: +1 650 625-2004

   Brian Haberman
   Caspian Networks
   1 Park Drive, Suite 300
   Research Triangle Park, NC  27709

   phone: +1-929-949-4828

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16.0 Change Log

   Draft <draft-ietf-ipv6-unique-local-addr-01.txt>

      o Removed example of PIR as an example of an allocation authority
        and clarified the text that the IANA should delegate the
        centrally assigned prefix to an allocation authority.
      o Changed sample code for generating pseudo random Global IDs to
        not require any human input.  Changes from using birthday to
        unique token (e.g., EUI-64, serial number, etc.)  available on
        machine running the algorithm.
      o Added a new section analyzing the uniqueness properties of the
        pseudo random number algorithm.
      o Added text to recommend that centrally assigned local addresses
        be used for site planning extensive inter-site communication.
      o Clarified that domain border routers should follow site border
        router recommendations.
      o Clarified that AAAA DNS records should not be installed in the
        global DNS.
      o Several editorial changes.

   Draft <draft-ietf-ipv6-unique-local-addr-00.txt>

      o Changed file name to become an IPv6 w.g. group document.
      o Clarified language in Routing and Firewall sections.
      o Several editorial changes.

   Draft <draft-hinden-ipv6-global-local-addr-02.txt>

      o Changed title and name of addresses defined in this document to
        "Unique Local IPv6 Unicast Addresses" with abbreviation of
        "Local IPv6 addresses".
      o Several editorial changes.

   Draft <draft-hinden-ipv6-global-local-addr-01.txt>

      o Added section on scope definition and updated application
        requirement section.
      o Clarified that, by default, the scope of these addresses is
      o Renumbered sections and general text improvements
      o Removed reserved global ID values
      o Added pseudo code for local allocation submitted by Brian
        Haberman and added him as an author.
      o Split Global ID values into centrally assigned and local
        assignments and added text to describe local assignments

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   Draft <draft-hinden-ipv6-global-local-addr-00.txt>

      o Initial Draft

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