IPv6 Expand / Compress

How this calculation works

An IPv6 address has eight groups of four hex digits, and two shortenings are permitted: leading zeros inside a group are dropped, and one run of all-zero groups may be replaced by a double colon. Expanding restores both. Compressing back is where implementations differ, and RFC 5952 settles it: replace the longest run of zeros rather than the first, never use :: more than once, and never compress a single zero group — that last rule exists because shortening one group saves nothing and makes the address harder to read back correctly. A trailing IPv4 form is handled too, since it is legal and becomes two hextets.

What the results mean

Expanded

All eight groups, four digits each. What you want when comparing addresses by eye or pasting into something strict.

Compressed

The canonical short form under RFC 5952.

::

Stands for one or more all-zero groups. Legal only once in an address.

Groups

The eight hextets separately, for checking a specific position.

Common problems and fixes

It rejects my address with two double colons

Only one :: is allowed. With two, the number of zero groups each stands for is ambiguous, so the address has no single meaning.

The compressed form differs from what I had

Yours may have compressed the first zero run rather than the longest, or compressed a single group. Both are valid to parse and neither is canonical.

My address ends in an IPv4 address

That form is legal and is expanded into the final two hextets. It appears in IPv4-mapped addresses.

Frequently asked questions

More in NetMath

• IPv4 to IPv6 MapperConvert an IPv4 address to its IPv6 representations.
• IP Subnet CalculatorAddress plus mask into network, broadcast, usable range and host count.