IPv6 Address Expander & Compressor

Understanding the IPv6 address expander and compressor — concept and principle

An IPv6 address is 128 bits long, written as eight groups of four hexadecimal digits separated by colons. Because that is long, the standard allows shortcuts: leading zeros in a group can be dropped, and one run of all-zero groups can be replaced by "::". This tool converts between the full (expanded) form and the canonical short form recommended by RFC 5952, and also builds the reverse-DNS name under ip6.arpa. It helps when comparing addresses across logs, writing firewall rules or creating PTR records.

Five written groups plus three hidden zero groups make the full eight.

How it is calculated

Expand: split the input at "::". Count the groups on each side; the missing groups (8 minus that count) are filled with 0000. Then pad every group with leading zeros to four digits.
Compress (RFC 5952): remove leading zeros in each group, write hex in lowercase, and replace the longest run of two or more consecutive zero groups with "::". If two runs are equally long, the first is shortened. A single zero group stays as "0".
Reverse DNS: take the 32 hex digits of the expanded form, reverse their order, separate each digit with a dot and append ".ip6.arpa".

The principle

"::" can appear only once, because the reader must be able to work out how many zero groups it stands for: 8 minus the groups that are written. RFC 4291 allows several spellings of one address; RFC 5952 picks one canonical spelling so text comparison and searching stay consistent. Reverse DNS works one hex digit (4 bits, a "nibble") at a time, which is why the ip6.arpa name has 32 labels.

Worked example

Input: 2001:db8::ff00:42:8329.
Left of "::" are 2 groups (2001, db8); right are 3 (ff00, 42, 8329). Missing: 8 − 5 = 3 zero groups.
Pad each group: 2001:0db8:0000:0000:0000:ff00:0042:8329.
Compressing again gives 2001:db8::ff00:42:8329, so the input was already canonical.
Reverse the 32 digits: 9.2.3.8.2.4.0.0.0.0.f.f.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa.

How to read the result

Use the expanded form for fixed-width storage and sorting, and the compressed form for display and documentation. The prefix 2001:db8::/32 is reserved for documentation (RFC 3849), so addresses like the example never appear on the real internet.

Common mistakes

Using "::" twice, which makes an address ambiguous and invalid.
Shortening a single zero group to "::"; RFC 5952 says to write it as 0.
Using uppercase hex in canonical text; RFC 5952 recommends lowercase.

Keep in mind: This tool accepts only hex groups and colons. Forms with an embedded IPv4 part (such as ::ffff:192.0.2.1) or a zone index (%eth0) are not parsed, and validity checking is basic.

Frequently asked questions

How many zeros does "::" represent?

As many full 16-bit zero groups as are needed to bring the total to eight.

Is 2001:0db8::1 the same address as 2001:db8::1?

Yes. Leading zeros inside a group do not change the value.

What is the ip6.arpa name used for?

It is the DNS name where a PTR record maps an IPv6 address back to a host name.

Sources: IETF RFC 4291, IP Version 6 Addressing Architecture · IETF RFC 5952, A Recommendation for IPv6 Address Text Representation · IETF RFC 3596, DNS Extensions to Support IP Version 6 · IETF RFC 3849, IPv6 Address Prefix Reserved for Documentation

Checked 8 Nov 2026. Standard formulas and RFC/IEEE definitions, computed locally in your browser — nothing you type is sent anywhere.

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