Paste an IPv6 address in expanded or compressed form to validate it and see both its fully expanded and RFC 5952 compressed versions.
The tool parses the address into eight 16-bit groups, expanding a double colon into the missing zero groups, then writes it back two ways. The expanded form pads every group to four hex digits. The compressed form follows RFC 5952: leading zeros are removed, the longest run of two or more zero groups (the first such run if tied) becomes a double colon, and letters are lower case. Embedded IPv4 endings such as ::ffff:192.0.2.1 are not accepted here; enter the address in hex groups.
What does :: mean in an IPv6 address? A double colon stands for one or more groups of zeros and can appear only once in an address. It is what makes compressed forms such as 2001:db8::1 possible.
Why is only the longest zero run compressed? RFC 5952 sets that rule so a given address has one recommended written form: compress the longest run of zero groups, use the first run when two tie, and never compress a single zero group.
Are upper-case letters allowed? Yes, hex letters A to F are accepted in either case on input. The compressed output uses lower case, as RFC 5952 recommends.