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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 6 (IPv6) / IP Next Generation (IPng)                      9  IPv6 Addressing

IPv6 Address Size and Address Space 1 2 3 4 IPv6 Address Space Allocation

Zero Compression in IPv6 Addresses

Fortunately, there is a short-cut that can be applied to shorten some addresses even further. This technique is sometimes called zero compression. The method allows a single string of contiguous zeroes in an IPv6 address to be replaced by a double-colon. So, for example, the address above could be expressed as:

805B:2D9D:DC28::FC57:D4C8:1FFF

We know how many zeroes are replaced by the “::” because we can see how many fully-expressed (“uncompressed”) hex words are in the address. In this case there are six, so the “::” represents two zero words. To prevent ambiguity, the double-colon can appear only once in any IP address, because if it appeared more than once we could not tell how many zeroes were replaced in each instance. So, if our example address were 805B:2D9D:DC28:0:0:FC57:0:0, we could replace either the first pair of zeroes or the second, but not both.

Zero compression doesn't make our example much shorter, but due to how IPv6 addresses are structured, long strings of zeroes are common. For example, consider this address:

FF00:4501:0:0:0:0:0:32

With compression, this could be expressed as just:

FF00:4501::32

It works even better on special addresses. The full IPv6 loopback address is:

0:0:0:0:0:0:0:1

With compression, this is simply:

::1

For even more fun, consider the Even more odd, the IPv6 “unspecified” address:

0:0:0:0:0:0:0:0

Apply zero compression to an address that is all zeroes, and what do you get? That’s right:

::

No numbers at all! Of course thinking of “::” as an address does take some getting used to.

IPv6 Address Size and Address Space 1 2 3 4 IPv6 Address Space Allocation