IPv6 Toolkit

Understanding IPv6 addresses

An IPv6 address is a 128-bit number, written as eight groups of four hexadecimal digits separated by colons, for example 2001:0db8:0000:0000:0000:ff00:0042:8329. The jump from IPv4's 32 bits to 128 bits was deliberate: 32 bits gives roughly 4.3 billion addresses, which the internet exhausted, whereas 128 bits gives about 3.4 × 10³⁸ — enough to assign a unique address to every device for the foreseeable future and to hand out huge blocks to every organisation without scarcity. That abundance is why subnetting in IPv6 feels so different: a single site is routinely given a /48, leaving 16 bits of subnet and a full 64-bit host portion per network.

Compressed versus expanded notation

The same address can be written two ways, and both are valid. The expanded form always shows all eight groups with their leading zeros, so it is unambiguous and lines up nicely in tables. The compressed form follows RFC 5952 and is what you should use in configuration and documentation. Three rules produce the canonical compressed form: write hex digits in lowercase; strip leading zeros from each group (0db8 becomes db8, 0000 becomes 0); and replace the single longest run of all-zero groups with a double colon (::). The :: may appear only once in an address, and when two runs of zeros are the same length you compress the first one. Applying those rules, the example above compresses to 2001:db8::ff00:42:8329. The Compress & Expand card does this in both directions and correctly handles a :: placed at the start, middle or end.

Global Unicast, Unique Local and Link-Local

Not every IPv6 address is meant to travel the public internet, and the leading bits tell you the scope:

• Global Unicast (2000::/3) — publicly routable addresses, the IPv6 equivalent of a public IPv4 address. Almost all addresses you see in the wild today start with 2 or 3.
• Unique Local, ULA (fc00::/7, in practice fd00::/8) — private addresses for use inside a site, similar in spirit to IPv4's 10.0.0.0/8. The 40-bit global ID is chosen randomly so two networks that later merge are very unlikely to collide. ULAs are not routed on the public internet.
• Link-Local (fe80::/10) — automatically configured on every IPv6 interface and valid only on the local link. They never cross a router and are used for neighbour discovery and as next-hop addresses.

How CIDR works in IPv6

CIDR notation in IPv6 is the same idea as in IPv4: a slash followed by the number of fixed leading bits, the prefix length. In 2001:db8::/48 the first 48 bits identify the network and the remaining 80 bits are free to address hosts and subnets within it. Because IPv6 has 128 bits, a block holds 2^{(128 − prefix)} addresses — numbers so large they are usually quoted as powers of two or in scientific notation. The network (first) address has all host bits set to zero and the last address has them all set to one; unlike IPv4 there is no reserved broadcast address, so every address in the range is usable. The CIDR Calculator card works all of this out, including a human-readable estimate of the block size.