Address Representation
You certainly already know that 32-bit IPv4 addresses are represented by breaking them into four 8-bit segments and writing each of those segments in decimal between 0 and 255, separating them with periods; hence the term dotted decimal.
128-bit IPv6 addresses are represented by breaking them up into eight 16-bit segments. Each segment is written in hexadecimal between 0x0000 and 0xFFFF, separated by colons. An example of a written IPv6 address is
3ffe:1944:0100:000a:0000:00bc:2500:0d0b
Remembering more than a few such addresses is practically impossible, and writing them is not much fun either. Fortunately, there are two rules for reducing the size of written IPv6 addresses. The first rule is
The leading zeroes in any 16-bit segment do not have to be written; if any 16-bit segment has fewer than four hexadecimal digits, it is assumed that the missing digits are leading zeroes.
In the example address, the third, fourth, fifth, sixth, and eighth segments have leading zeroes. Using the first address compression rule, the address can be written as
3ffe:1944:100:a:0:bc:2500:d0b
Notice that only leading zeroes can be omitted; trailing zeroes cannot, because doing so would make the segment ambiguous. You would not be able to tell whether the missing zeroes belonged before or after the written digits.
Notice also that the fifth segment in the example address is all zeroes, and is written with a single zero. Many IPv6 addresses have long strings of zeroes in them. Take, for example, the following address:
ff02:0000:0000:0000:0000:0000:0000:0005
This address can be reduced as follows:
ff02:0:0:0:0:0:0:5
However, using the second rule can reduce this address even further:
Any single, contiguous string of one or more 16-bit segments consisting of all zeroes can be represented with a double colon.
Using this rule, the example address can be represented as the following:
ff02::5
The increased convenience in writing such an address is obvious. But notice that the rule says only a single contiguous string of all-zero segments can be represented with a double colon. Using the double colon more than once in an IPv6 address can create ambiguity. Take, for example, the following address:
2001:0d02:0000:0000:0014:0000:0000:0095
Either of the following reductions of the address is correct because they use a double colon only once:
2001:d02::14:0:0:95
2001:d02:0:0:14::95
But the following reduction is illegal because it uses the double colon twice:
2001:d02::14::95
It is illegal because the length of the two all-zero strings is ambiguous; it could represent any of the following IPv6 addresses:
2001:0d02:0000:0000:0014:0000:0000:0095
2001:0d02:0000:0000:0000:0014:0000:0095
2001:0d02:0000:0014:0000:0000:0000:0095
Unlike IPv4, in which the prefixthe network portion of the addresscan be identified by a dotted decimal or hexadecimal address mask or a bitcount, IPv6 prefixes are always identified by bitcount. That is, the address is followed by a forward slash and a decimal number indicating how many of the first bits of the address are the prefix bits. For example, the prefix of the following address is the first 64 bits:
3ffe:1944:100:a::bc:2500:d0b/64
When you are writing just an IPv6 prefix, you set all the host bits to 0 the same way you do with IPv4 addresses. For example
3ffe:1944:100:a::/64
An IPv6 address consisting of all zeroes can be written simply with a double colon. There are two cases where an all-zeroes address is used. The first is a default address, in which the address is all zeroes and the prefix length is zero:
::/0
The second all-zeroes IPv6 address is an unspecified address, which is used in some Neighbor Discovery Protocol procedures described later in this chapter. An unspecified address is a filler, indicating the absence of a real IPv6 address. When writing an unspecified address, it is differentiated from a default address by its prefix length:
::/128
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