DNS on Windows Server 2003, 3rd Edition

The ABCs of IPv6 Addressing

The Microsoft DNS Server supports the new AAAA record necessary for IPv6. We thought some information on the representation and structure of IPv6 addresses would be appropriate to help you understand the record.

IPv6 addresses are 128 bits long. The preferred representation of an IPv6 address is eight groups of as many as four hexadecimal digits, separated by colons. For example:

0123:4567:89ab:cdef:0123:4567:89ab:cdef

The first group of hex digits (0123, in this example) represents the most significant (or highest order) four bits of the address.

Groups of digits that begin with one or more zeros don’t need to be padded to four places, so you can also write the previous address as:

123:4567:89ab:cdef:123:4567:89ab:cdef

Each group must contain at least one digit, though, unless you’re using the :: notation. The :: notation allows you to compress sequential groups of zeros. This comes in handy when you’re specifying just an IPv6 prefix. For example, this notation:

dead:beef::

specifies the first 32 bits of an IPv6 address as dead:beef and the remaining 96 as zeros.

You can also use :: at the beginning of an IPv6 address to specify a suffix. For example, the IPv6 loopback address is commonly written as:

::1

or 127 zeros followed by a single one. You can even use :: in the middle of an address as a shorthand for contiguous groups of zeros:

dead:beef::1

You can use the :: shorthand only once in an address since more than one could be ambiguous.

IPv6 prefixes are specified ...