TCP/IP Tutorial and Technical Overview - IBM Redbooks

Global unicast address format
IPv6 unicast addresses are aggregatable with prefixes of arbitrary bit-length,
similar to IPv4 addresses under Classless Inter-Domain Routing.
The latest global unicast address format, as specified in RFC 3587 – IPv6
Address architecture and RFC 4291 – IPv6 Global Unicast Address Format, is
expected to become the predominant format used for IPv6 nodes connected to
the Internet.
Note: This note is intended for readers who worked on the previous unicast
format. For new readers, you can skip this special note.
The historical IPv6 unicast address used a two-level allocation scheme which
has been replaced by a coordinated allocation policy defined by the Regional
Internet Registries (RIRs). There are two reasons for this major change:
► Part of the motivation for obsoleting the old TLA/NLA structure is technical;
for instance, there is concern that TLA/NLA is not the technically best
approach at this stage of the deployment of IPv6.
► Another part of the reason for new allocation of IPv6 addresses is related
to policy and to the stewardship of the IP address space and routing table
size, which the RIRs have been managing for IPv4.
The Subnet Local Aggregator (SLA) field in the original UNicast Address
Structure remains in function, but with a different name called “subnet ID,”
which we describe later in the unicast address format.
Figure 9-8 shows the general format for IPv6 global unicast addresses.
< n bits > < m bits > < 128-n-m bits >
Global Routing Prefix Subnet ID Interface ID
Figure 9-8 Global unicast address format
Where:
Global Routing Prefix
A value assigned to a site for a cluster of subnets/links.
The global routing prefix is designed to be structured
hierarchically by the RIRs and ISPs.
342 TCP/IP Tutorial and Technical Overview