An IPv6 address is represented as eight groups of four hexadecimal digits, each group representing 16 bits[a] The groups are separated by colons (:). An example of an IPv6 address is:
2001:0db8:85a3:0000:0000:8a2e:0370:7334
The standards provide flexibility in the representation of IPv6 addresses. The full representation of eight four-digit groups may be simplified by several techniques, eliminating parts of the representation. In general, representations are shortened as much as possible. However, this practice complicates several common operations, namely searching for a specific address or an address pattern in text documents or streams, and comparing addresses to determine equivalence. For mitigation of these complications, the IETF has defined a canonical format for rendering IPv6 addresses in text:[8]
  • The hexadecimal digits are always compared in case-insensitive manner, but IETF recommendations suggest the use of only lower case letters. For example, 2001:db8::1 is preferred over 2001:DB8::1;
  • Leading zeros in each 16-bit field are suppressed, but each group must retain at least one digit. For example, 2001:0db8::0001:0000 is rendered as 2001:db8::1:0;
  • The longest sequence of consecutive all-zero fields is replaced with two colons (::). If the address contains multiple runs of all-zero fields of the same size, to prevent ambiguities, it is the leftmost that is compressed. For example, 2001:db8:0:0:1:0:0:1 is rendered as 2001:db8::1:0:0:1 rather than as 2001:db8:0:0:1::1. :: is not used to represent just a single all-zero field. For example, 2001:db8:0:0:0:0:2:1 is shortened to 2001:db8::2:1, but 2001:db8:0000:1:1:1:1:1 is rendered as 2001:db8:0:1:1:1:1:1.
These methods can lead to very short representations for IPv6 addresses. For example, the localhost (loopback) address, 0:0:0:0:0:0:0:1, and the IPv6 unspecified address, 0:0:0:0:0:0:0:0, are reduced to ::1 and ::, respectively.
During the transition of the Internet from IPv4 to IPv6, it is typical to operate in a mixed addressing environment. For such use cases, a special notation has been introduced, which expresses IPv4-mapped and IPv4-compatible IPv6 addresses by writing the least-significant 32 bits of an address in the familiar IPv4 dot-decimal notation, whereas the 96 most-significant bits are written in IPv6 format. For example, the IPv4-mapped IPv6 address ::ffff:c000:0280 is written as ::ffff:192.0.2.128, thus expressing clearly the original IPv4 address that was mapped to IPv6.