Aggregate Global Unicast Addresses

Aggregate global unicast addresses are used for global communication. These addresses are similar in function to IPv4 addresses under classless interdomain routing (CIDR). The following table shows their format.

Table 14–2 Aggregate Global Unicast Addresses Format

3 bits	13 bits	8 bits	24 bits	16 bits	64 bits
FP	TLA ID	RES	NLA ID	SLA ID	Interface ID

FP	Format Prefix (001)
TLA ID	Top-Level Aggregation identifier
RES	Reserved for future use
NLA ID	Next-Level Aggregation identifier
SLA ID	Site-Level Aggregation identifier
INTERFACE ID	Interface identifier

The first 48 bits represent the public topology. The next 16 bits represent the site topology.

The first 3 bits identify the address as an aggregate global unicast address. The next field, TLA ID, is the top level in the routing hierarchy. The next 8 bits are reserved for future use. The NLA ID field is used by organizations that are assigned a TLA ID to create an addressing hierarchy and to identify sites.

The SLA ID field is used by an individual organization to create its own local addressing hierarchy and to identify subnets. Use of the SLA ID field is analogous to subnets in IPv4 except that each organization has a much greater number of subnets. The 16–bit SLA ID field supports 65,535 individual subnets. The Interface ID is used to identify interfaces on a link. The Interface ID must be unique on that link. The Interface ID can also be unique over a broader scope. In many instances, an interface identifier is the same or is based on the interface's link-layer address.