ULA

Unique Local Addresses (ULA)

There are also unique local addresses with a prefix fc00::/7, these will not travel through the Internet and have significance only on a local site. Confusingly, the 8th bit must be set to 1 so all the addresses appear as fd00::/8. They are a little like RFC1918 addresses used for NAT on an IPv4 network.

The next 40 bits are made up for your organisation (example; AB CDEF 1234) and should be truly random. If you need to join together two organisations, the address ranges should have been completely different, and you can do so without renumbering. RFC4193 specifies how these addresses should be generated and many tools exist on the internet to create a random allocation.

The next 16 bits are for hierarchical subnetting within the organisation; typically, 4 bits for site, 4 bits for building, 8 bits for subnets.

ULA can be used with one-to-one NAT such as NPTv6 so that sites can have unique internal addressing which does not need to change after changing providers. ULA are a handy way to get started with IPv6 even without an external connection of a GUA allocation.