One might suppose that the most obvious difference between IPv4 and IPv6 is the address space: IPv4 addresses are only 32 bits long, and IPv6 addresses are 128 bits long. And there are more differences in the way IPv6 uses those addresses. However, in another sense, address length will be the least obvious change because it will be invisible to most users. Applications carried over IPv6 networks must rely completely on the Domain Name System (DNS) to correctly link IP host names to IPv6 addresses and networks. DNS and IPv6 will be discussed in detail in Chapter 18.

In this chapter, we begin with a discussion of the IPv6 addressing architecture, as designed by the original IPng working groups. Next, we look at the IPv6 address space, address formats, and the way in which addresses are intended to be allocated. IPv6 does away with broadcasts and relies instead on unicast and multicast, and adds a new category: anycast addresses.

The IPv6 addressing architecture was first published as a proposed standard in 1995, in RFC 1884, IP Version 6 Addressing Architecture. As originally formulated, IPv6 addresses were defined to be 128-bit identifiers for interfaces and sets of interfaces. Three types of addresses unicast, multicast, and anycast were defined, with unicast defined as an identifier for a single interface. A packet sent to a unicast address is delivered to the interface identified by that address. Multicast and anycast addressing will be discussed later in this chapter and...