IPv6 address: expand and shorten

Why an IPv6 address has two forms

A full IPv6 address is 32 hex digits in eight groups of four, for example 2001:0db8:0000:0000:0000:ff00:0042:8329. That is long to read and easy to mistype, so there is an agreed short form. Computers accept both. Trouble starts when people compare two addresses that are the same but written differently, or when a program expects one particular form. This tool gives both.

The rules for shortening

1. Drop leading zeros in each group. 0db8 becomes db8, 0042 becomes 42, and 0000 becomes 0.
2. Replace one run of all-zero groups with ::. In the example, three zero groups in a row collapse: 2001:db8::ff00:42:8329.
3. Use :: only once. If it appeared twice, nobody could tell how many zero groups each one stood for.
4. Pick the longest run. If two runs are equally long, shorten the first.
5. Do not use :: for a single zero group. Write that group as 0.
6. Use small letters: 2001:db8::1, not 2001:DB8::1.

Rules 4 to 6 come from the internet standard on writing IPv6 addresses (RFC 5952), which exists so that everyone produces the same short form for the same address. The tool follows it.

The rule for expanding

Count the groups that are written. There must be eight in all, so :: stands for however many are missing. Then pad every group to four digits with leading zeros. 2001:db8::1 has three groups written, so :: stands for five groups of zeros: 2001:0db8:0000:0000:0000:0000:0000:0001.

What else the tool shows

• Kind of address: global (public), unique local (private), link-local, multicast, loopback, documentation and a few rarer kinds.
• The first 64 bits: in a normal IPv6 network the first half of the address names the network and the second half names the device. The network part is what your provider or router assigned.

Things that trip people up

• Zone names: on your own computer a link-local address may appear as fe80::1%eth0 or fe80::1%12. The part after the percent sign names the network port. It is not part of the address, and the tool ignores it.
• Square brackets: in a web address an IPv6 address is wrapped in brackets so the colons are not mistaken for a port: http://[2001:db8::1]:8080/. The tool accepts the brackets.
• IPv4 at the end: forms such as ::ffff:192.168.1.1 are valid. The last two groups are written as an IPv4 address.
• Letters beyond f: hex has only 0 to 9 and a to f. A "g" or an "o" means a typing slip.

Try these

Paste fe80:0000:0000:0000:0202:b3ff:fe1e:8329, ::1, 2001:db8:0:0:1:0:0:1 and ff02::1 to see different kinds and how the longest run is chosen.