Global Unicast Address

Global Unicast Address
The IPv6 aggregatable global unicast address is the equivalent to the Class A, B, or C IPv4 address.
Theoretically, a global unicast address is any address that is not one of the other named types,
which accounts for 85 percent of the IPv6 address space. But IANA has been limited to allocating
only aggregatable global unicast addresses, which begin with binary 001, a portion of the address
known as the global unicast format prefix, which is 2000::/3 in IPv6 hexadecimal notation. This
is still the largest block of assigned IPv6 addresses and represents 1⁄8 of the total address space.
The structure of global unicast addresses enables aggregation of the routing prefixes that will
limit the number of routing table entries in the global routing table. Global unicast addresses are
aggregated upward through an organization and eventually to the Internet service providers
(ISPs). Figure 2.7 shows that global unicast addresses, which start with binary 001, are made up
of a global routing prefix, followed by a subnet ID, and finally an interface ID.