Issues Associated with Designating Additional Private IPv4 Address Space
RFC 6319

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Internet Engineering Task Force (IETF)                        M. Azinger
Request for Comments: 6319                       Frontier Communications
Category: Informational                                      Corporation
ISSN: 2070-1721                                                L. Vegoda
                                                               July 2011

                         Issues Associated with
           Designating Additional Private IPv4 Address Space


   When a private network or internetwork grows very large, it is
   sometimes not possible to address all interfaces using private IPv4
   address space because there are not enough addresses.  This document
   describes the problems faced by those networks, the available
   options, and the issues involved in assigning a new block of private
   IPv4 address space.

   While this informational document does not make a recommendation for
   action, it documents the issues surrounding the various options that
   have been considered.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Large Networks . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Non-Unique Addresses . . . . . . . . . . . . . . . . . . . . .  3
     3.1.  Subscriber Use Network Address Translation . . . . . . . .  3
     3.2.  Carrier-Grade Network Address Translation  . . . . . . . .  4
   4.  Available Options  . . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  IPv6 Options . . . . . . . . . . . . . . . . . . . . . . .  4
       4.1.1.  Unique Globally Scoped IPv6 Unicast Addresses  . . . .  4
       4.1.2.  Unique Local IPv6 Unicast Addresses  . . . . . . . . .  5
     4.2.  IPv4 Options . . . . . . . . . . . . . . . . . . . . . . .  5
       4.2.1.  Address Transfers or Leases from Organizations
               with Available Address Space . . . . . . . . . . . . .  5
       4.2.2.  Using Unannounced Address Space Allocated to
               Another Organization . . . . . . . . . . . . . . . . .  5
       4.2.3.  Unique IPv4 Space Registered by an RIR . . . . . . . .  6
   5.  Options and Consequences for Defining New Private Use Space  .  6
     5.1.  Redefining Existing Unicast Space as Private Address
           Space  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     5.2.  Unique IPv4 Space Shared by a Group of Operators . . . . .  7
     5.3.  Potential Consequences of Not Redefining Existing
           Unicast Space as Private Address Space . . . . . . . . . .  8
     5.4.  Redefining Future Use Space as Unicast Address Space . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     7.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   [RFC1918] sets aside three blocks of IPv4 address space for use in
   private networks:, and
   These blocks can be used simultaneously in multiple, separately
   managed networks without registration or coordination with IANA or
   any Internet registry.  Very large networks can find that they need
   to number more device interfaces than there are available addresses
   in these three ranges.  It has occasionally been suggested that
   additional private IPv4 address space should be reserved for use by
   these networks.  Although such an action might address some of the
   needs for these very large network operators, it is not without
   consequences, particularly as we near the date when the IANA free
   pool will be fully allocated.

   The overall conclusion is that allocating additional address space to
   be used as private address space has severe problems and would, for
   instance, impact any software or configuration that has built-in
   assumptions about private address space.  However, it is also well
   understood that cascading Network Address Translation (NAT)
   deployments in the existing private address space will cause
   different types of severe problems when address spaces overlap.  At
   this point, there is no clear agreement of the likelihood of various
   problems or the respective trade-offs.

2.  Large Networks

   The main categories of very large networks using private address
   space are: cable operators, wireless (cell phone) operators, private
   internets, and VPN service providers.  In the case of the first two
   categories, the complete address space reserved in [RFC1918] tends to
   be used by a single organization.  In the case of private internets
   and VPN service providers, there are multiple independently managed
   and operated networks and the difficulty is in avoiding address

3.  Non-Unique Addresses

3.1.  Subscriber Use Network Address Translation

   The address space set aside in [RFC1918] is a finite resource that
   can be used to provide limited Internet access via NAT.  A discussion
   of the advantages and disadvantages of NATs is outside the scope of
   this document, but an analysis of the advantages, disadvantages, and
   architectural implications can be found in [RFC2993].  Nonetheless,
   it must be acknowledged that NAT is adequate in some situations and
   not in others.  For instance, it might technically be feasible to use
   NAT or even multiple layers of NAT within the networks operated by

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   residential users or corporations where only limited Internet access
   is required.  A more detailed analysis can be found in [RFC3022].
   Where true peer-to-peer communication is needed or where services or
   applications do not work properly behind NAT, globally unique address
   space is required.  In other cases, NAT traversal techniques
   facilitate peer-to-peer like communication for devices behind NATs.

   In many cases, it is possible to use multiple layers of NAT to re-use
   parts of the address space defined in [RFC1918].  It is not always
   possible to rely on Customer Premises Equipment (CPE) devices using
   any particular range, however.  In some cases, this means that
   unorthodox workarounds including assigning CPE devices unallocated
   address space or address space allocated to other network operators
   are feasible.  In other cases, organizations choose to operate
   multiple separate routing domains to allow them to re-use the same
   private address ranges in multiple contexts.  One consequence of this
   is the added complexity involved in identifying which system is
   referred to when an IP address is identified in a log or management

3.2.  Carrier-Grade Network Address Translation

   Another option is to share one address across multiple interfaces and
   in some cases, subscribers.  This model breaks the classical model
   used for logging address assignments and creates significant risks
   and additional burdens, as described in [CLAYTON] and more fully
   discussed in [FORD], and as documented in [DS-LITE].

4.  Available Options

   When a network operator has exhausted the private address space set
   aside in [RFC1918] but needs to continue operating a single routing
   domain, a number of options are available.  These are described in
   the following sections.

4.1.  IPv6 Options

4.1.1.  Unique Globally Scoped IPv6 Unicast Addresses

   Using unique, globally scoped IPv6 unicast addresses is the best
   permanent solution as it removes any concerns about address scarcity
   within the next few decades.  Implementing IPv6 is a major endeavor
   for service providers with millions of consumers and is likely to
   take considerable effort and time.  In some cases, implementing a new
   network protocol on a very large network takes more time than is
   available, based on network growth and the proportion of private
   space that has already been used.  In these cases, there is a call

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   for additional private address space that can be shared by all
   network operators.  [DAVIES] makes one such case.

4.1.2.  Unique Local IPv6 Unicast Addresses

   Using the unique, local IPv6 unicast addresses defined in [RFC4193]
   is another approach and does not require coordination with an
   Internet registry.  Although the addresses defined in [RFC4193] are
   probabilistically unique, network operators on private internets and
   those providing VPN services might not want to use them because there
   is a very low probability of non-unique locally assigned global IDs
   being generated by the algorithm.  Also, in the case of private
   internets, it can be very challenging to coordinate the introduction
   of a new network protocol to support the internet's continued growth.

4.2.  IPv4 Options

4.2.1.  Address Transfers or Leases from Organizations with Available
        Address Space

   The Regional Internet Registry (RIR) communities have recently been
   developing policies to allow organizations with available address
   space to transfer such designated space to other organizations
   [RIR-POLICY].  In other cases, leases might be arranged.  This
   approach is only viable for operators of very large networks if
   enough address space is made available for transfer or lease and if
   the very large networks are able to pay the costs of these transfers.
   It is not possible to know how much address space will become
   available in this way, when it will be available, and how much it
   will cost.  However, it is unlikely to become available in large
   contiguous blocks, and this would add to the network management
   burden for the operator as a significant number of small prefixes
   would inflate the size of the operators routing table at a time when
   it is also adding an IPv6 routing table.  These reasons will make
   address transfers a less attractive proposition to many large network
   operators.  Leases might not be attractive to some organizations if
   both parties cannot agree to a suitable length of time.  Also, the
   lessor might worry about its own unanticipated needs for additional
   IPv4 address space.

4.2.2.  Using Unannounced Address Space Allocated to Another

   Some network operators have considered using IP address space that is
   allocated to another organization but is not publicly visible in BGP
   routing tables.  This option is very strongly discouraged as the fact
   that an address block is not visible from one view does not mean that
   it is not visible from another.  Furthermore, address usage tends to

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   leak beyond private network borders in e-mail headers, DNS queries,
   traceroute output and other ways.  The ambiguity this causes is
   problematic for multiple organizations.  This issue is discussed in
   [RFC3879], Section 2.3.

   It is also possible that the registrant of the address block might
   want to increase its visibility to other networks in the future,
   causing problems for anyone using it unofficially.  In some cases,
   there might also be legal risks involved in using address space
   officially allocated to another organization.

   Where this has happened in the past, it has caused operational
   problems [FASTWEB].

4.2.3.  Unique IPv4 Space Registered by an RIR

   RIRs' policies allow network operators to receive unique IP addresses
   for use on internal networks.  Further, network operators are not
   required to have already exhausted the private address space set
   aside in [RFC1918].  Nonetheless, network operators are naturally
   disinclined to request unique IPv4 addresses for the private areas of
   their networks, as using addresses in this way means they are not
   available for use by new Internet user connections.

   It is likely to become more difficult for network operators to obtain
   large blocks of unique address space as we approach the point where
   all IPv4 unicast /8s have been allocated.  Several RIRs already have
   policies about how to allocate from their last /8
   [RIR-POLICY-FINAL-8], and there have been policy discussions that
   would reduce the maximum allocation size available to network
   operators [MAX-ALLOC] or would reduce the period of need for which
   the RIR can allocate [SHORTER-PERIODS].

5.  Options and Consequences for Defining New Private Use Space

5.1.  Redefining Existing Unicast Space as Private Address Space

   It is possible to re-designate a portion of the current global
   unicast IPv4 address space as private unicast address space.  Doing
   this could benefit a number of operators of large networks for the
   short period before they complete their IPv6 roll-out.  However, this
   benefit incurs a cost by reducing the pool of global unicast
   addresses available to users in general.

   When discussing re-designating a portion of the current global
   unicast IPv4 address space as private unicast address space, it is
   important to consider how much space would be used and for how long
   it would be sufficient.  Not all of the large networks making full

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   use of the space defined in [RFC1918] would have their needs met with
   a single /8.  In 2005, [HAIN] suggested reserving three /8s for this
   purpose, while in 2009 [DAVIES] suggested a single /10 would be
   sufficient.  There does not seem to be a consensus for a particular
   prefix length nor an agreed basis for deciding what is sufficient.
   The problem is exacerbated by the continually changing needs of ever
   expanding networks.

   A further consideration is which of the currently unallocated IPv4
   unicast /8 blocks should be used for this purpose.  Using address
   space that is known to be used unofficially is tempting.  For
   instance,, which was unallocated until January 2010, was
   proposed in [HAIN] and is known to be used by a number of different
   users.  These include networks making use of HIP LSIs [RFC4423],
   [WIANA], [anoNet], and others.  There is anecdotal [VEGODA] and
   research [WESSELS] evidence to suggest that several other IPv4 /8s
   are used in this fashion.  Also there have been discussions [NANOG]
   about some sections of these /8's being carved out and filtered,
   therefore unofficially enabling the use of these sections for private

   Although new IPv4 /8s are allocated approximately once a month, they
   are not easy to bring into use because network operators are slow to
   change their filter configurations.  This is despite long-running
   awareness campaigns [CYMRU] [LEWIS] and active work [ripe-351] to
   notify people whose filters are not changed in a timely fashion.
   Updating code that recognizes private address space in deployed
   software and infrastructure systems is likely to be far more
   difficult as many systems have these ranges hard-coded and cannot be
   quickly changed with a new configuration file.

   Another consideration when redefining existing unicast space as
   private address space is that no single class of user can expect the
   space to stay unique to them.  This means that an ISP using a new
   private address range cannot expect its customers not to already be
   using that address range within their own networks.

5.2.  Unique IPv4 Space Shared by a Group of Operators

   Where a group of networks find themselves in a position where they
   each need a large amount of IPv4 address space from an RIR in
   addition to that defined in [RFC1918], they might cooperatively agree
   to all use the same address space to number their networks.  The
   clear benefit to this approach is that it significantly reduces the
   potential demand on the pool of unallocated IPv4 address space.
   However, the issues discussed in Sections 4.2.2 and 5.3 are of
   concern here, particularly the possibility that one operator might

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   decide to use the address space to number customer connections,
   rather than private infrastructure.

   Nonetheless, this approach has the potential to create an unofficial
   new private address range without proper scrutiny.

5.3.  Potential Consequences of Not Redefining Existing Unicast Space as
      Private Address Space

   If additional private address space is not defined and the large
   network operators affected by this problem are not able to solve
   their problems with IPv6 address space or by segmenting their
   networks into multiple routing domains, those networks will need
   unique IPv4 addresses.  It is possible and even likely that a single
   network could consume a whole IPv4 /8 in a year.  At the time this
   document is being written, there are just 24 unallocated IPv4 /8s, so
   it would not take many such requests to make a major dent in the
   available IPv4 address space.  [POTAROO] provides an analysis of IPv4
   address consumption and projects the date on which the IANA and RIR
   pools will be fully allocated.

5.4.  Redefining Future Use Space as Unicast Address Space

   There have also been proposals to re-designate the former Class E
   space ( as unicast address space.  [WILSON] suggests that
   it should be privately scoped while [FULLER] does not propose a
   scope.  Both proposals note that existing deployed equipment may not
   be able to use addresses from  Potential users would
   need to be sure of the status of the equipment on their network and
   the networks with which they intend to communicate.

   It is not immediately clear how useful could be in
   practice.  While [FULLER] documents the status of several popular
   desktop and server operating systems, the status of the most widely
   deployed routers and switches is less clear, and it is possible that might only be useful in very large, new green field
   deployments where full control of all deployed systems is available.
   However, in such cases it might well be easier to deploy an IPv6

6.  Security Considerations

   This document has no security implications.

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7.  References

7.1.  Normative References

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

   [RFC2993]     Hain, T., "Architectural Implications of NAT",
                 RFC 2993, November 2000.

   [RFC3022]     Srisuresh, P. and K. Egevang, "Traditional IP Network
                 Address Translator (Traditional NAT)", RFC 3022,
                 January 2001.

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

7.2.  Informative References

   [CLAYTON]     Clayton, R., "Practical mobile Internet access
                 traceability", January 2010,

   [CYMRU]       Greene, B., "The Bogon Reference",

   [DAVIES]      Davies, G. and C. Liljenstolpe, "Transitional
                 non-conflicting reusable IPv4 address block", Work
                 in Progress, November 2009.

   [DS-LITE]     Durand, A., Droms, R., Woodyatt, J., and Y.  Lee,
                 "Dual-Stack Lite Broadband Deployments Following IPv4
                 Exhaustion", Work in Progress, August 2010.

   [FASTWEB]     Aina, A., "41/8 announcement", May 2006,

   [FORD]        Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
                 Roberts, "Issues with IP Address Sharing", Work
                 in Progress, March 2010.

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   [FULLER]      Fuller, V., Lear, E., and D. Meyer, "Reclassifying
                 240/4 as usable unicast address space", Work
                 in Progress, March 2008.

   [HAIN]        Hain, T., "Expanded Address Allocation for Private
                 Internets", Work in Progress, January 2005.

   [LEWIS]       Lewis, J., "This system has been setup for testing
                 purposes for 69/8 address space", March 2003,

   [MAX-ALLOC]   Spenceley, J. and J. Martin, "prop-070: Maximum IPv4
                 allocation size", January 2009,

   [NANOG]       Dickson, B., "1/8 and 27/8 allocated to APNIC",
                 January 2010, <

   [POTAROO]     Huston, G., "IPv4 Address Report",

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

   [RFC4423]     Moskowitz, R. and P. Nikander, "Host Identity Protocol
                 (HIP) Architecture", RFC 4423, May 2006.

   [RIR-POLICY]  Number Resource Organization, "RIR Comparative Policy
                 Overview, October 2009, Section 1.3.2 Transfer of

                 Number Resource Organization, "RIR Comparative Policy
                 Overview, October 2009, 2.6. Use of Final Unallocated
                 IPv4 Address Space", October 2009, <

                 Karrenberg, D., O'Reilly, N., Titley, N., and R. Bush,
                 "RIPE Policy Proposal 2009-03", April 2009,
                 < proposals/2009-03>.

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   [VEGODA]      Vegoda, L., "Awkward /8 Assignments", September 2007,

   [WESSELS]     Wessels, D., "Searching for Evidence of Unallocated
                 Address Space Usage in DITL 2008 Data", June 2008,

   [WIANA]       WIANA, "The Wireless Internet Assigned Numbers
                 Authority", <>.

   [WILSON]      Wilson, P., Michaelson, G., and G. Huston,
                 "Redesignation of 240/4 from "Future Use" to "Private
                 Use"", Work in Progress, September 2008.

   [anoNet]      anoNet, "anoNet: Cooperative Chaos".

   [ripe-351]    Karrenberg, D., "De-Bogonising New Address Blocks",
                 October 2005, <>.

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Appendix A.  Acknowledgments

   The authors would like to thank Ron Bonica, Michelle Cotton, Lee
   Howard, and Barbara Roseman for their assistance in early discussions
   of this document and to Maria Blackmore, Alex Bligh, Mat Ford, Thomas
   Narten, and Ricardo Patara for suggested improvements.

Authors' Addresses

   Marla Azinger
   Frontier Communications Corporation
   Vancouver, WA
   United States of America


   Leo Vegoda
   Internet Corporation for Assigned Names and Numbers
   4676 Admiralty Way, Suite 330
   Marina del Rey, CA  90292
   United States of America

   Phone: +1-310-823-9358

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