What indicates a unique local address in IPv6?

A unique local address in IPv6 is indicated by the prefix that starts with the hexadecimal digits 'FD'. Unique local addresses are used for local communications within a site or between a limited set of sites and are not routable on the public Internet. The full range for these addresses begins with 'FD00::/8'.

The designation allows for site-local addressing, giving network administrators the flexibility to manage addresses without the need for global uniqueness in the same manner as public IPv6 addresses. This addressing scheme is particularly useful for organizations that wish to implement private networks but still leverage IPv6 capabilities.

In contrast, other hexadecimal prefixes such as 'FA', 'FB', and 'FC' do not represent unique local addresses and instead are not defined for standard use in IPv6 addressing. Thus, the uniqueness of the local address is specifically associated with the 'FD' prefix, making it the correct indicator for unique local addresses in the IPv6 protocol.