[1] These addresses are non-globally reachable[2] (routable only within the scope of private networks, but not the global IPv6 Internet).
However, insufficient definition of the term site led to confusion over the governing routing rules.
In September 2004, the Internet Engineering Task Force (IETF) deprecated the definition of this address range,[4] and postulated solutions to its problems.
The special behaviour for this type of addresses, as required at that time,[5] was lifted in 2006 and the block returned to regular global unicast.
They are not mathematically guaranteed to be globally unique, but the probability of a collision is nevertheless extremely small.
However, if networks require routing ULAs between each other in the event of a merger, for example, the risk of address collision is very small if the RFC 4193 selection algorithm was used.
One such example is Amazon Web Services, which uses ULAs within its virtual private cloud networking.