IPv6 unique local addresses begin with which two hexadecimal digits?

IPv6 unique local addresses (ULAs) are designed for local communications within a site and are not routable on the global Internet. These addresses allow for private addressing similar to IPv4 private addresses in the ranges 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.

Unique local addresses always begin with the prefix 'FD', which signifies that the address is intended for local use. The 'FC' prefix is also reserved for local use, but it is not commonly utilized within practice as 'FD'. This means that while both 'FC' and 'FD' are valid local address prefixes, the commonly recognized and specified prefix for unique local addresses is 'FD'.

The other prefixes listed—'FE' and 'FF'—do not apply to unique local addresses. 'FE' is typically associated with reserved addresses, such as those used for multicast, and 'FF' is used for anycast and multicast addresses, which serve different purposes altogether.

Thus, when identifying IPv6 unique local addresses, the correct identification is made with the prefix 'FD'.