The whole concept of global unicast addressing does have many similarities as compared with IPv4. If viewing IPv4 addresses from a classless perspective, both IPv4 and IPv6 global unicast addresses have two parts: subnet plus host for IPv4 and prefix plus interface ID for IPv6. The format of the addresses commonly list a slash followed by the prefix length–a convention sometimes referred to as CIDR notation, and other times as prefix notation. Subnetting works much the same, with a public prefix assigned by some num-bering authority, and the Enterprise choosing subnet numbers, extending the length of the prefix to make room to number the subnets.
IPv6 addressing, however, includes several other types of unicast IPv6 addresses beside the global unicast address. Additionally, IPv6 defines other general categories of ad-dresses, as summarized in this list.
■ Unicast: Like IPv4, hosts and routers assign these IP addresses to a single interface for the purpose of allowing that one host or interface to send and receive IP packets.
■ Multicast: Like IPv4, these addresses represent a dynamic group of hosts, allowing a host to send one packet that is then delivered to every host in the multicast group.
IPv6 defines some special-purpose multicast addresses for overhead functions (such as NDP). IPv6 also defines ranges of multicast addresses for application use.
■ Anycast: This address type allows the implementation of a nearest server among du-plicate servers concept. This design choice allows servers that support the exact same function to use the exact same unicast IP address. The routers then forward a packet destined for such an address to the nearest server that is using the address.
Two big differences exist when comparing general address categories for IPv4 and IPv6.
First, IPv6 adds the formal concept of Anycast IPv6 addresses as shown in the preceding list. IPv4 does not formally define an Anycast IP address concept, although a similar con- cept may be implemented in practice.
Second, IPv6 simply has no Layer 3 broadcast ad-dresses. For example, all IPv6 routing protocols send Updates either to Unicast or Multicast IPv6 addresses, and overhead protocols such as NDP make use of multicasts as well. In IPv4, ARP still uses broadcasts, and older routing protocols such as RIP-1 also used broadcasts.With IPv6, there is no need to calculate a subnet broadcast address (hoorah!) and no need to make hosts process overhead broadcast packets meant only for a
few devices in a subnet.
Finally, note that IPv6 hosts and router interfaces typically have at least two IPv6 ad- dresses and may well have more. Hosts and routers typically have a Link Local type of IPv6 address. A router may or may not have a global unicast address, and may well have multiple. IPv6 simply allows the configuration of multiple IPv6 addresses with no need for or concept of secondary IP addressing.
