So, the RFC for “Unique Local IPv6 Unicast Addressing” has finally shown up as RFC 4193. These addresses are intended to cover some of the ground originally provided for by IPv6 site-local addressing. They also help out people who aren’t in a position to get IPv6 addresses from an ISP, maybe because they don’t have one.
The idea is to provide a large block of addresses that look global, are allocated randomly but that we all agree to not route globally. This means that they are cheap (generating random numbers is cheap), they are stable (because they don’t depend on your ISP), they work just like globals (so you don’t need to change your applications) and they can even be used site-to-site or when sites merge (because randomly allocating them is highly unlikely to produce collisions). However, because we do not know how to manage a routing table without aggregation, we can not route them globally.
Operationally, people won’t need to make many changes to support them. We’ll probably want to filter for these addresses at our site boundaries, and may want to make our recursive DNS servers authoritative for the reverse domain, in the same way people should answer for 10.in-addr.arpa if they use a subset of 10.0.0.0/8 for private addressing.