IPv6 tutorial – Part 4: IPv6 address syntax

In this article, you will learn the main concepts of the IPv6 address syntax: colon-hexadecimal representation, leading zero suppression, zero compression, and IPv6 prefix.

Now that you know about the new features of IPv6, it is time to have a closer look at the practical details. In this post, I will give a short summary about the IPv6 address syntax. It is essentially a condensed version of the corresponding part in Microsoft’s white paper “Introduction to IP Version 6.”

Colon-hexadecimal representation

An IPv6 address consists of 128 bits and is presented in eight 16-bit blocks. Each 16-bit block is converted to a four-digit hexadecimal number. Blocks are separated by colons.

Example: 2001:0DB8:0000:2F3B:02AA:00FF:FE28:9C5A

Leading zero suppression

Because IPv6 addresses are quite long, the leading zeroes within a 16-bit block can be removed, but each block must have at least a single digit.

Example: 2001:DB8:0:2F3B:2AA:FF:FE28:9C5A

Zero compression

A contiguous sequence of 16-bit blocks set to 0 can be replaced with the so-called double colon (::). Zero compression can only be applied once in an IP address. To determine how many blocks have been omitted, you just have to count the remaining blocks and subtract this number from 8.

Example: FE80:0:0:0:2AA:FF:FE9A:4CA2 can be zero compressed to FE80::2AA:FF:FE9A:4CA2.

IPv6 prefix

IPv6 prefixes are used to express IPv6 subnets, routes, and address ranges. The syntax of IPv6 prefixes looks like this: address/prefix-length. It is comparable to the Classless Inter-Domain Routing (CIDR) notation for IPv4 (for instance, 192.168.0.0/16 represents a Class B subnet): Subnet masks are no longer used in IPv6.

Example: 21DA:D3:0:2F3B::/64 represents a subnet of 2⁶⁴ addresses, where the first 64 bits are fixed and the last 64 bits are variable.

Admittedly, IPv6 addresses look somewhat complicated compared to the relatively simple IPv4 addresses. Rest assured that typos in IPv6 addresses will knock down quite a few systems once IPv6 starts replacing IPv4. But this is the price of the large address space. I guess, we will get used to it. In my next post, I will discuss the different types of IPv6 addresses. I promise that things won’t get easier.

By Michael Pietroforte

[IPv6 – Part 1]  [IPv6 – Part 2]  [IPv6 – Part 3]  [IPv6 – Part 4]  [IPv6 – Part 5]  [IPv6 – Part 6]