How to shorten an IPv6 address

The full address

An IPv6 address is 128 bits, written as eight groups of four hexadecimal digits separated by colons. Each group (sometimes called a hextet) holds 16 bits. Written out in full, an address looks like this:

2001:0db8:0000:0000:0000:0000:0000:0010

That's 39 characters, and a lot of them are zeros. Nobody wants to type that, so the standards allow two shortcuts.

Rule 1: drop the leading zeros

In each group, you can leave out the zeros at the front. The value doesn't change, the same way 007 is just 7.

• 0db8 becomes db8
• 0000 becomes 0 (a group always keeps at least one digit)
• 0010 becomes 10

Only leading zeros go. Trailing zeros stay: 0010 is 10, never 1. After rule 1, our address is 2001:db8:0:0:0:0:0:10.

Rule 2: one run of zero groups becomes ::

Next, find a run of consecutive all-zero groups and replace the whole run with a double colon (::). Our address has five zero groups in a row, so they vanish in one move:

2001:db8:0:0:0:0:0:10 becomes 2001:db8::10

That's 12 characters instead of 39. To expand it again, count the groups you can see (three here) and fill the :: with enough zero groups to make eight (five).

Only one double colon

You can use :: once per address. With two, there's no way to tell how many zero groups each one stands for. 2001::1::10 could mean 2001:0:1:0:0:0:0:10 or 2001:0:0:0:0:1:0:10, so it isn't a valid address.

When an address has more than one run of zeros, RFC 5952 says to compress the longest run, and if two runs tie, the first one. Write the hex digits in lowercase, and don't use :: for a single zero group.

Try it yourself

On a computer with IPv6, look at your own addresses:

• Windows: run ipconfig and look for "IPv6 Address" or "Link-local IPv6 Address".
• Mac or Linux: run ifconfig or ip -6 addr.

Most will start with fe80::, a link-local address that's already been shortened with ::. Try expanding one back to eight full groups.

Good to know

• Only leading zeros can be dropped; trailing zeros stay (0010 → 10, never 1).
• RFC 5952 recommends lowercase hex and compressing the longest run of zero groups (the first one if two runs tie), and not using :: for a single zero group.
• 2001:0db8:0000:0000:0000:0000:0000:0010 (39 characters) → 2001:db8::10 (12 characters).

For CCNA learners

This lines up with CCNA 200-301 exam topic 1.8, configure and verify IPv6 addressing and prefix: be ready to shorten a full IPv6 address, expand a shortened one back to eight groups, and spot an invalid address with two ::.

Answer to the comment question

2001:db8:1::1 hides four all-zero groups. An IPv6 address always has eight groups, and four are written out (2001, db8, 1 and 1), so the :: stands for the other four. In full it's 2001:0db8:0001:0000:0000:0000:0000:0001.