Unique local address

A unique local address (ULA) is an IPv6 non-globally-routable subnet identifier, as defined in RFC 4193. They are supposed to be used for systems that are not connected to the Internet. They can be used for instance as address space in a factory, allowing the use for IPv6 for every machine.

History

In 2003, RFC 3513 defined fec0::/10 as site-local addresses, that could be used within a "site" as a private IPv6 network. However, this led to confusion as to what constituted a "site," and how such a network would be routed (ala RFC 1918 IPv4 networks). RFC 3879 in September 2004 deprecated this address range, and postulated solutions to the problems it had. In October 2005, RFC 4193 was drafted, allocating a subnet of fc00::/7 for use in private IPv6 networks and VPNs. They are non-routable in global IPv6 network. They are routable only inside of a private network or between a limited set of sites.

Allocation

fc00::/7 is divided into two /8 address groups, assigned and random, from which valid /48 subnet prefixes are derived.

*fc00::/8 is to be managed by a so-called "ULA-Central" for assigned /48s. As of 9-2007, this body has yet to be formed.
*fd00::/8 is allocated by appending a randomly-generated 40-bit string, to derive a valid /48. [http://tools.ietf.org/html/rfc4193 RFC4193] offers a suggestion on how to do the random generation, to try to guarantee a minimum-quality result if the user does not have access to a good source of random numbers.

Issues

*There is an extremely small chance that two generated prefixes are the same. See the RFC for the risk calculation.
*reverse DNS entries (under ip6.arpa) for ULAs cannot be delegated in the global DNS as there is no central registration system. Also ULA is supposed to be used locally as such should not be tied to the global DNS. Tying it to the global DNS system would require hosts to be able to talk to the global DNS system and thus would either require hosts to have both a global and ULA address or force the use of NAT.
*In the absence of a "ULA-Central" authority, SixXS has created a [http://www.sixxs.net/tools/grh/ula/ voluntary database] to avoid address conflicts. If you use this registry then conflicts are even less likely to occur.

External links

* [http://noc.sixxs.net/tools/grh/ula/ SixXS public ULA registration database]
* [http://www.hznet.de/tools/generate-rfc4193-addr A tool to create unique local ipv6 unicast adresses]
* [http://www.arin.net/meetings/minutes/ARIN_XIII/PDF/Monday/IPv6_Narten.pdf ULA draft proposal from 2004]

References

*ISBN 3-540-24524-3: "IPv6 in Practice" by Benedikt Stockebrand, 2006