Publisher Summary This chapter begins with a discussion of the IPv6 addressing architecture, as designed by the original IPng working groups. The most obvious difference between IPv4 and IPv6 is in their addressing formats. IPv4 uses 32-bit (4-byte) addresses to uniquely identify nodes within the global Internet. IPv6 uses 128-bit (16-byte) addresses to uniquely identify nodes within the global Internet. It also discusses the IPv6 address space, address formats, and the way in which addresses are intended to be allocated. IPv6 does away with broadcasts, relies instead on unicast and multicast and adds a new category: anycast addresses. Three types of addresses unicast, multicast, and anycast are discussed, and unicast is 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 is discussed in the chapter.