Predictions of the date when the current Internet Protocol Version 4 (IPv4) 32 bit address space will be exhausted have turned out to be premature, but the rapid proliferation of Internet connected devices such as cell phones and PDAs have accelerated the need for the transition to Internet Protocol Version 6 (IPv6). IPv6 uses 128 bit addresses to provide ample addresses far into the future. An understanding of address formats is the first step in learning how to prepare your network for IPv6. This is part one of a two part series describing IPv6 address formats and how they are used.
The designers of IPv6 took advantage of the opportunity to design a new protocol to do more than simply expand the address space. They structured the new addresses to enable nodes to configure automatically, to reduce the effort required to renumber a network, and to increase the efficiency of Internet backbone routers. Recognizing that the transition from IPv4 to IPv6 will take many years, the designers provided formats to enable IPv4 and IPv6 to coexist. IPv6 addresses are defined by RFC 3513, Internet Protocol Version 6 (IPv6) Addressing Architecture.
There are three types of IPv6 addresses and each address type can be classified by the part of the network over which it is valid. In addition, there is an address type used in the transition from IPv4 to IPv6. As in the case of IPv4, a network interface may support multiple addresses at the same time. It may support addresses of each of the various types and may support multiple addresses of the same type.
IPv6 Address Types
Unicast: Unicast addresses are identical to IPv4 unicast addresses. A packet sent to a unicast address will be received by only one network node.
Multicast: Multicast addresses are used in the same way as multicast addresses in IPv4. Multicast was added to IPv4 after the basic address formats were designed. The result is that multicast addresses are confined to a very limited portion of the IPv4 address space. In IPv6, the multicast format will provide ample space for multicast addresses.
Anycast: Anycast addresses are new in IPv6. They are shared by multiple nodes. A message sent to an anycast address is sent to only one of the interfaces identified by the anycast address. For example, all of the DNS servers at a site may share a single anycast address since all provide identical service. A DNS request sent to that address will go to whichever server is closest to the requesting node in terms of the site's internal routing protocol.
IPv4 Mapped: IPv4 addresses are used to support the IPv4 to IPv6 transition.
Scope of Address Validity
Link local addresses: Link local addresses are valid only on a single link. Packets carrying link local addresses cannot be routed to another link.
Site local addresses: Site local addresses are valid within a site. Packets carrying them cannot be routed from the site.
Global addresses: Global addresses are unique across the Internet. A packet must contain a global address to be routed outside the local network in which it originates.
Link local and site local addresses replace IPv4's private addresses. They provide the extra flexibility of confining some packets to a single link but allowing other packets to travel within a site but not to leave the site.
http://searchnetworking.techtarget.com/infoCenter/tip/0,294276,sid7_gci1083238_iid2651,00.html described how IPv6 designers have carefully created these address types to support facilities that ease network management and enable efficient Internet operation for years to come.
David B. Jacobs has more than twenty years of networking industry experience. He has managed leading-edge software development projects and consulted to Fortune 500 companies as well as software start-ups.
