IPv6 addresses for private networks

Yesterday's IPv6 deployment session ran over slightly, so there was no time to discuss a question Christian Chu Fook of Grenada's GD Management Group, asked. Nonetheless, it's a good question and touches on an active area of discussion in the IETF's IPv6 WG.

is there a scope within IPv6, like IPv4, that is reserved for private networks?

Initially, a /10 IPv6 prefix was reserved for Site Local addressing but this was deprecated in September 2004 and replaced with Unique Local Addresses (ULAs) which are defined in RFC 4193. This RFC sets aside a /8 IPv6 prefix from which /48 prefixes can be pseudo-randomly selected. Although it sounds like randomly selecting an address prefix could lead to an address clash at some point it is actually remarkably unlikely. Even if a site using a generated ULA prefix connected to 999 other sites using randomly generated ULA prefixes the chance of an address clash would be incredibly small: 4.54*10^-05

Nonetheless, some people want unique local addresses that come from a central registry as they believe this will remove all possibility of address clashes. The proposal can be found on the IETF web site and discussion is taking place in the IETF's IPv6 WG.