Acronym for Unique Local Adressess
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: http://people.nokia.net/hinden/draft-ietf-ipv6-unique-local-addr-09.txt
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