Redesignation of 240/4 from "Future Use" to "Private Use"
Asia Pacific Network Information
Centre
pwilson@apnic.net
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Asia Pacific Network Information
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ggm@apnic.net
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Asia Pacific Network Information
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gih@apnic.net
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Internet Area
Individual Submission
This document directs the IANA to designate the block of IPv4
addresses from 240.0.0.0 to 255.255.255.255 (240.0.0.0/4) as
unicast address space for Private Use.
This document directs the IANA to designate the block of IPv4
addresses from 240.0.0.0 to 255.255.255.255 (240.0.0.0/4) as
unicast address space for Private Use.
The address block spanning 240.0.0.0 to 255.255.255.255
(240.0.0.0/4), formerly designated as "Class E", and noted as
being "Reserved" in the IANA IPv4 address registry, is no longer
to be held in reserve by IANA for the IETF.
IANA is directed to redesignate the address block 240.0.0.0/4
as unicast address space intended for Private Use. While the
particular form of private use is not specified here, it is
envisaged that this address prefix would have use in large
private Internets that require more address space than is
available in the private use address space designated by during the dual stack transition to
IPv6.
Potential users of this address space need to ensure that
their envisaged deployment can satisfy the caveats noted
here.
Many implementations of the TCP/IP protocol stack have the
240.0.0.0/4 address block marked as experimental, and prevent
the host from forwarding IP packets with addresses drawn from
this address block.
For this reason, it is strongly suggested that private
network addressing requirements which can be fulfilled from the
private use address space designated by should continue to use that space.
Network administrators with very large scale requirements for
private use address space who wish to use addresses drawn from
240.0.0.0/4 are advised to conduct appropriate tests to ensure
that such addresses can be used in their envisaged private use
context.
[Note: not for publication. It is suggested that in order to
assist with verification of equipment compatibility, a separate
informational RFC or other mechanism be developed to assist with
the recording of specific test results, upgrade status, etc.]
Note: This section is to assist in the discussion of the
recommendation proposed in this draft. It is intended that this
section would be removed prior to publication.
The option of using this "top part" of the IPv4 address space as a
means of mitigating to some extent the issues related to the
exhaustion of the IPv4 unallocated address pool date back to at
least 1998, if not earlier .
A related internet-draft, ,
advocates changing the designation of this addres prefix to that
of a "useable" unicast address block, without specifying whether
the designation should be for private or public use, so the
"reserved" status was proposed in this draft for the
240.0.0.0/address block. This proposal differs from in advocating that the address block not
be used for publically routed address space, but is to be limited
to private use contexts. The reason for this decision to propose a
designation of Private Use is that for public use the entire
installed base of IPv4 hosts un the public Internet, as well as
associated private Internet realms that are attached to the public
internet via NATs need to be able to generate and forward IPv4
packets that are addressed to 240.0.0.0/4 addresses. The set of
changes to host systems may not be undertaken to a generally
useful extent within any reasonable timeframe. The alternative
approach is to limit its intended useof 240.0.0.0/4 to private
network realms where the population of end devices and forwarding
systems that need to support the use of 240.0.0.0/4 address space
is limited. In private use contexts the utility of using this
space in a private context is a local decision that is not
impacted by any external factors of private use elsewhere.
It has been noted that many end host operating system protocol
stacks do not support the use of 240.0.0.0/4 address
space. However, reported in March 2008
that: "Apple OSX has been confirmed to support the use
of 240.0.0.0/4 as unicast address space. Changes have been
incorporated into recent versions of Sun Solaris and have been
submitted for inclusion in the Linux kernel tree. No plans have
been announced for modifications to any version of Microsoft
Windows, in part because of uncertainty over how to perform 6-to-4
tunneling in the absence of a definitive statement on whether
240.0.0.0/4 is "public" or "private" space.
This draft advocates the adoption of a definitive statement in the
IPv4 address registry that 240.0.0.0/4 is Private Use space to
allow transitonal tunnelling mechanisms to perform correctly in
the context of use of 6to4 , Teredo , and similar forms of IPv6 transitional
mechanisms that use IPv6 tunnelling as an overlay on an IPv4
substrate.
It has been commented that this draft requires a similar level
of effort in terms of deployment overheads to that involved in the
deployment of IPv6 itself. This observation has been used to argue
against adopting this proposal and instead reiterate the calls for
the adoption of IPv6 and avoid any unnecessary distaction of
effort in articifially prolonging the useful lifespan of IPv4. In
response to such arguments it is noted that the adoption of IPv6
in an orderly transition context requires the exctended use of
dual stack support in networks where both IPv6 and IPv4 is
available for use. The problem is that the transition phase is now
anticipated to last for far longer than the remaining lifetime of
the unallocated address pool of IPv4 addresses, and in supporting
the dual stack IPv6 transition, there is a need for additional
IPv4 addresses in any case. The 240.0.0.0/4 address block allows
service provider infrastructure to be numbered in a manner that
would not conflict with either customer private address space use
from space, or public address space.
This private use address pool is intended to assist in the IPv6
transition of larger networks who are using IPv4 in the context of
a dual stack deployment. In such contexts it is reported to be the
case that the reuse of network 10.0.0/8 is not an option because
of existing use and potential address clashes . The use of 240.0.0.0/4 offers a more
conventional method to interconnect CPE NATs and network border carrier
NATs without having to use more involved solutions such as or .
Equipment deployed on the public Internet is configured by
default to treat addresses in the block 240.0.0.0/4 as
experimental addresses that cannot be forwarded. This implies
that accidental leakage of packets destined to such addresses
would conventionally be discarded.
The IANA is directed to redesignate the block of IPv4
addresses from 240.0.0.0 to 255.255.255.255 as unicast address
space reserved for "Private Use".
The authors would like to acknowledge the thoughtful assistance
of David Conrad, Andy Davidson and Robert Seastrom in the
preparation of this document.
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