Acronym for Unique Local Adressess

IPv6's answer to RFC:1918. ULA addresses are reasonably unique but only locally routable and are designed to replace SiteLocal addresses (which were deprecated in RFC:3879)

There are two alternative specifications, each allocated to one half of ULA space. One has people randomly allocate addresses (in FD00::/8) in a way that makes it statistically improbable that two people will generate the same address and then want to communicate. The second way has the addresses (in FC00::/8) assigned "free" by a central authority thus preventing two people from using the same addresses.

The current draft ULA draft:

This has been announced as RFC:4193 Unique Local IPv6 Unicast Addresses