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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 6 (IPv6) / IP Next Generation (IPng)                      9  IPv6 Addressing

IPv6 Addressing 1 2 IPv6 Address Size and Address Space

IPv6 Address Types

One important change in the addressing model of IPv6 is the address types supported. IPv4 supported three address types: unicast, multicast and broadcast. Of these, the vast majority of actual traffic was unicast. IP multicast support was not widely deployed until many years after the Internet was established, and continues to be hampered by various issues. Use of broadcast in IP had to be severely restricted for performance reasons (we don't want any device to be able to broadcast across the entire Internet!)

IPv6 also supports three address types, but with some changes:

• Unicast Addresses: These are standard unicast addresses as in IPv4, one per host interface.
  
  
• Multicast Addresses: These are addresses that represent various groups of IP devices: a message sent to a multicast address goes to all devices in the group. IPv6 includes much better multicast features and many more multicast addresses than IPv4. Since multicast under IPv4 was hampered in large part due to lack of support of the feature by many hardware devices, support for multicasting is a required, not optional, part of IPv6.
  
  
• Anycast Addresses: Anycast addressing is used when a message must be sent to any member of a group, but does not need to be sent to them all. Usually the member of the group that is easiest to reach will be sent the message. One common example of how anycast addressing could be used is in load sharing amongst a group of routers in an organization.

Broadcast addressing as a distinct addressing method is gone in IPv6. Broadcast functionality is implemented using multicast addressing to groups of devices. A multicast group to which all nodes belong can be used for broadcasting in a network, for example.

An important implication of the creation of anycast addressing is removal of the strict uniqueness requirement for IP addresses. Anycast is accomplished by assigning the same IP address to more than one device. The devices must also be specifically told that they are sharing an anycast address, but the addresses themselves are structurally the same as unicast addresses.

The bulk of the remainder of this section focuses on unicast addressing, since it is by far the most important type. Multicast and anycast addressing are given special attention separately.

IPv6 Addressing 1 2 IPv6 Address Size and Address Space