What IPv6 address block is reserved for unique local addresses?

The address block reserved for unique local addresses in IPv6 is FC00::/7. This range is specifically intended for use in local communications within a site and is not routable on the global Internet, similar to the IPv4 private address ranges (like 192.168.x.x, 10.x.x.x, etc.).

The use of unique local addresses facilitates internal networking and allows organizations to create their own network configurations without concern for address collisions in the global IPv6 space, since these addresses are meant to be used internally only.

In contrast, the other options serve different purposes: FE80::/10 is designated for link-local addresses, which are used for communication within a single network segment; 2000::/3 is the global unicast address space, meant for routable IP addresses on the Internet; and ::1/128 is the loopback address, used to refer to the local system itself. Each of these address types has specific applications that do not overlap with the purpose of unique local addresses.