Planning IPv6 Addressing

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

To plan an efficient IPv6 addressing strategy, you must understand how IPv6 addressing works. IPv6 addressing is a major departure from IPv4 addressing. The most obvious difference is that IPv4 uses 4-byte source and destination addresses, typically expressed in the familiar dotted-decimal notation, whereas IPv6 uses 16-byte addresses, typically expressed in colon-hexadecimal notation. Colon-hexadecimal notation uses eight 4-digit hexadecimal numbers, with colons separating the 16-bit blocks (the 4-digit numbers).

To manage addresses more easily, IPv6 suppresses leading zeros and compresses a single contiguous all-zero 16-bit block, representing the contiguous block with two colons (::) (known as double-colon compression). Table 1.6 shows the effects of suppressing leading zeros and double-colon compression on the notation for an IPv6 address.

Table 1.6   Leading Zero Suppression and All-Zero Contiguous Block Compression

The 16 bytes, or 128 bits, provided in the IPv6 address space potentially supports 2128 addresses. However, the purpose of this large address space is not only to provide an inexhaustible supply of addresses, but also to enable a hierarchical routing infrastructure that can be summarized. IPv6 addressing is designed to minimize the size of routing tables and to reduce routing complexity.

IPv6 supports address configuration both in the presence of a DHCP server, known as stateful address configuration, and in the absence of a DHCP server, known as stateless address configuration. Stateless address configuration introduces the use of link-local addresses, whereby hosts on the same link automatically configure themselves with IPv6 addresses for that link and can use those addresses to communicate with the other hosts on the same link. If one or more local routers exist, hosts can use router discovery to automatically determine the routers’ addresses and can then communicate with IPv6 hosts beyond the local link.

As in IPv4, the high-order bits in an IPv6 address identify the type of address. In IPv6, the high-order bits are known as the Format Prefix (FP). IPv6 does not use subnet masks to specify the network ID. Instead, it uses only prefix notation.