A unique local address (ULA) is an IPv6 address in the block fc00::/7 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.
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History
In 2003, RFC 3513 defined the block fec0::/10 as site-local addresses, that could be used within a "site" for private IPv6 networks. However, insufficient definition of the term site lead to confusion over the resulting routing rules. RFC 3879 (September 2004) deprecated this address range, and postulated solutions to its problems. In October 2005, RFC 4193 was published, reserving the address block fc00::/7 for use in private IPv6 networks. They are not routable in the global IPv6 Internet. They are routable only within a private network or between a limited set of sites.
Definition
fc00::/7 is divided into two /8 address groups, assigned and random, from which valid /48 prefixes are derived.
- fc00::/8 is to be managed by a so-called "ULA-Central" for /48s in use. As of September 2007[update], this body has yet to be formed.
- fd00::/8 is formed by appending a randomly-generated 40-bit string, to derive a valid /48 block. RFC 4193 offers a suggestion on how to generate the random identifier 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. The RFC document outlines the risk calculation.
- reverse Domain Name System (DNS) entries (under ip6.arpa) for ULAs cannot be delegated in the global DNS as there is no central registration system. ULA is supposed to be used locally as such should not be tied to the global DNS. Tying it to the global DNS would require hosts to be able to talk to the global DNS and thus would either require hosts to have both a global and ULA address or force the use of network address translation.
- In the absence of a "ULA-Central" authority, SixXS has created a voluntary database to avoid address conflicts.
References
- ISBN 3-540-24524-3: IPv6 in Practice by Benedikt Stockebrand, 2006
External links
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