IPv6 Essentials

Format Prefixes

RFC 2373 lists a number of format prefixes (also called global routing prefixes) that are used to identify special addresses, such as link-local addresses or multicast addresses. Table 3-2 outlines the initial assignment of reserved prefixes. The major part of the address space (over 80 percent) is unassigned, which leaves room for future assignments.

Table 3-2. List of assigned prefixes

Allocation	Prefix binary	Prefix hex	Fraction of address space
Reserved	0000 0000	::0/128	1/256
Reserved for NSAP allocation Reserved for IPX allocation (deprecated in later draft)	0000 001 0000 010		1/128 1/128
Aggregatable global unicast addresses	001		1/8
Link-local unicast addresses Site-local unicast addresses	1111 1110 10 1111 1110 11	FE80::/10 FEC0::/10	1/1024 1/1024
Multicast addresses	1111 1111	FF00::/8	1/256

Some special addresses are assigned out of the reserved address space with the binary prefix 0000 0000. These include the unspecified address, the loopback address, and IPv6 addresses with embedded IPv4 addresses, which will be discussed in detail later in this chapter. In drafts released after RFC 2373, the prefix for IPX has been removed. (The most recent draft, at the time of writing, is available at http://www.ietf.org/internet-drafts/draft-ietf-ipngwg-addr-arch-v3-08.txt.)

Unicast addresses can be distinguished from multicast addresses by their prefix. Globally unique unicast addresses have a high-order byte starting with 001. An IPv6 ...