Section 3.2. Address Types

3.2. Address Types

IPv4 knows unicast, broadcast, and multicast addresses. With IPv6, the broadcast address is not used anymore; multicast addresses are used instead. This is good news because broadcasts are a problem in most networks. The anycast address, a new type of address introduced with RFC 1546, has already been used in the IPv4 world but will probably be used on a wider basis with IPv6.

3.2.1. Unicast, Multicast, and Anycast Addresses

An IPv6 address can be classified into one of three categories:

Unicast

A unicast address uniquely identifies an interface of an IPv6 node. A packet sent to a unicast address is delivered to the interface identified by that address.

Multicast

A multicast address identifies a group of IPv6 interfaces. A packet sent to a multicast address is processed by all members of the multicast group.

Anycast

An anycast address is assigned to multiple interfaces (usually on multiple nodes). A packet sent to an anycast address is delivered to only one of these interfaces, usually the nearest one.

3.2.2. Some General Rules

IPv6 addresses are assigned to interfaces as in IPv4, not to nodes as in OSI, so each interface of a node needs at least one unicast address. A single interface can also be assigned multiple IPv6 addresses of any type (unicast, multicast, and anycast). A node can therefore be identified by the address of any of its interfaces. It is also possible to assign one unicast address to multiple interfaces for load-sharing reasons, but if you do this, you need to make sure that the hardware and drivers support it.

With IPv6, all zeros and ones are legal values for any field in an address.

IPv6 supports addresses of different scopes . There are global and non-global (e.g., link-local) scopes. Operationally, the use of non-global addresses has been introduced with IPv4 by using IP addresses from the private range or administratively scoped multicast addresses. The design of IPv6 includes the address scope in the base architecture. Every IPv6 address other than the unspecified address has a specific scope, which is a topological span within which the address may be used as a unique identifier for an interface or set of interfaces. The scope of an address is encoded as part of the address. You can find a description of scopes in the "Multicast Address" section later in this chapter, and refer to RFC 4007, "IPv6 Scoped Address Architecture" for an explanation of scopes.