Internet Engineering Task Force (IETF) M. Azinger
Request for Comments: 6319 Frontier Communications
Category: Informational Corporation
ISSN: 2070-1721 L. Vegoda
ICANN
July 2011
Issues Associated with
Designating Additional Private IPv4 Address Space
Abstract
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
http://www.rfc-editor.org/info/rfc6319.
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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
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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: 192.168.0.0/16, 172.16.0.0/12 and 10.0.0.0/8.
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
clashes.
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
system.
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
Organization
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, 1.0.0.0/8, 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
use.
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 (240.0.0.0/4) 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 240.0.0.0/4. 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 240.0.0.0/4 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
240.0.0.0/4 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
network.
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,
<http://www.lightbluetouchpaper.org/
2010/01/13/practical-mobile-internet-access-
traceability/>.
[CYMRU] Greene, B., "The Bogon Reference",
<http://www.team-cymru.org/Services/Bogons/>.
[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,
<http://www.afnog.org/archives/2006-May/002117.html>.
[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,
<http://69box.atlantic.net/>.
[MAX-ALLOC] Spenceley, J. and J. Martin, "prop-070: Maximum IPv4
allocation size", January 2009,
<http://www.apnic.net/policy/proposals/prop-070>.
[NANOG] Dickson, B., "1/8 and 27/8 allocated to APNIC",
January 2010, <http://mailman.nanog.org/
pipermail/nanog/2010-January/017451.html>.
[POTAROO] Huston, G., "IPv4 Address Report",
<http://www.potaroo.net/tools/ipv4/index.html>.
[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
Custodianship",
<http://www.nro.net/rir-comparative-policy-overview/
rir-comparative-policy-overview-2009-03#1-3-2>.
[RIR-POLICY-FINAL-8]
Number Resource Organization, "RIR Comparative Policy
Overview, October 2009, 2.6. Use of Final Unallocated
IPv4 Address Space", October 2009, <http://www.nro.net/
rir-comparative-policy-overview/
rir-comparative-policy-overview-2009-03>.
[SHORTER-PERIODS]
Karrenberg, D., O'Reilly, N., Titley, N., and R. Bush,
"RIPE Policy Proposal 2009-03", April 2009,
<http://www.ripe.net/ripe/policies/ proposals/2009-03>.
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[VEGODA] Vegoda, L., "Awkward /8 Assignments", September 2007,
<http://www.cisco.com/web/about/ac123/ac147/
archived_issues/ipj_10-3/103_awkward.html>.
[WESSELS] Wessels, D., "Searching for Evidence of Unallocated
Address Space Usage in DITL 2008 Data", June 2008,
<https://www.dns-oarc.net/files/dnsops-2008/
Wessels-Unused-space.pdf>.
[WIANA] WIANA, "The Wireless Internet Assigned Numbers
Authority", <http://www.wiana.org/>.
[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, <http://www.ripe.net/ripe/docs/ripe-351>.
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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
EMail: marla.azinger@ftr.com
URI: http://www.frontiercorp.com/
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
EMail: leo.vegoda@icann.org
URI: http://www.iana.org/
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