Textual representation of IPv6 addresses

IPv4 addresses are represented in dotted decimal format. The 32-bit address is divided along 8-bit boundaries. Each set of 8 bits is converted to its decimal equivalent and separated by periods. In contrast, IPv6 addresses are 128 bits divided along 16-bit boundaries. Each 16-bit block is converted to a 4-digit hexadecimal number and separated by colons. The resulting representation is called colon-hexadecimal.

The following forms are the three conventional forms for representing IPv6 addresses as text strings:

The preferred form is x:x:x:x:x:x:x:x, where the x's are the hexadecimal values of the eight 16-bit pieces of the address. For example:
```
2001:DB8:7654:3210:FEDC:BA98:7654:3210
```
```
2001:DB8:0:0:8:800:200C:417A
```
Guideline: You do not need to write the leading zeros in an individual field, but there must be at least one numeral in every field (except for the case described in the following item).

As a result of some methods of allocating certain styles of IPv6 addresses, sometimes addresses contain long strings of zero bits. To make writing addresses containing zero bits easier, a special syntax is available to compress the zeros. A double colon (::) indicates multiple groups of 16 bits of zeros and can appear only once in an address. The double colon can also be used to compress both leading and trailing zeros in an address.

For example the following addresses:

Table 1. Address types
Address type	Long form	Compressed form
Unicast	2001:DB8:0:0:8:800:200C:417A	2001:DB8::8:800:200C:417A
Multicast	FF01:0:0:0:0:0:0:101	FF01::101
Loopback	0:0:0:0:0:0:0:1	::1
Unspecified	0:0:0:0:0:0:0:0	::

An alternative form that is sometimes more convenient when dealing with a mixed environment of IPv4 and IPv6 nodes is x:x:x:x:x:x:d.d.d.d, where the x's are the hexadecimal values of the six high-order 16-bit pieces of the address, and the d's are the decimal values of the four low-order 8-bit pieces of the address (standard IPv4 representation). This form is used for IPv4-mapped IPv6 addresses. This type of address is used to hold an embedded IPv4 address. The address can be expressed in the following manner:
```
0:0:0:0:0:FFFF:129.144.52.38
```
The address can also be expressed in compressed form:
```
::FFFF:129.144.52.38
```