Unique local IPv6 addresses have the same function as IPv4 private addresses. They are not allocated by an address registry and are not meant to be routed outside their domain. Unique local IPv6 addresses begin with FD00::/8.
A unique local IPv6 address is constructed by appending a randomly generated 40-bit hexadecimal string to the FD00::/8 prefix. The subnet field and interface ID are created in the same way as with global IPv6 addresses.
A graphical representation of an unique local IPv6 address:
NOTE – The original IPv6 RFCs defined a private address class called site local. This class has been deprecated and replaced with unique local addresses.