7 IPv6 Addressing

Address structure and notation

An IPv6 address is a $128$-bit identifier assigned to an interface. It is written as eight groups of up to four hexadecimal digits separated by colons. For example, 2001:0db8:0000:0000:0000:0000:0000:0042 consists of eight $16$-bit groups.

Shortening IPv6 addresses

IPv6 notation can be shortened in two ways:

• Omit leading zeros in any group. For example, 0042 becomes 42.

• Replace one consecutive run of zero groups with ::. Use :: at most once because it represents however many zero groups are needed to make eight groups. For example, 2001:db8::42 expands to 2001:db8:0:0:0:0:0:42.

The recommended Canonical form uses lowercase letters, omits leading zeros, and compresses the longest run of zero groups. If multiple runs are tied for longest, choose the first. Do not use :: to replace only one zero group.

IPv6 address types

IPv6 has three fundamental address types:

• Unicast identifies one interface, and a packet is delivered to that interface. Common categories include global unicast, intended for globally routed communication; link-local (fe80::/10), used on a local link; and unique-local (fc00::/7), intended for local communications within sites or organizations.

• Anycast identifies a set of interfaces. A packet is routed to one member of the set, typically the nearest according to routing.

• Multicast identifies a group of interfaces, and a packet is delivered to the group. Multicast addresses are in ff00::/8.

IPv6 has no broadcast addresses; multicast provides group-delivery functions. The unspecified address is ::, and the loopback address is ::1. These are special addresses, not ordinary host addresses.

Prefix lengths and address boundaries

A Prefix length states how many leftmost bits identify a network prefix. It follows a slash: 2001:db8:1234:5600::/56 has a $56$-bit prefix, so the remaining $72$ bits are outside that prefix.

The prefix /n means that the first $n$ bits are fixed as the network prefix. The remaining bits are calculated as:

For example, /64 leaves $64$ bits outside the prefix. The address 2001:db8:1234:5600::42/64 belongs to 2001:db8:1234:5600::/64: the first four $16$-bit groups are the $64$-bit prefix, and the final four groups are outside it.

In many unicast networks, an address is considered as a Subnet prefix followed by an Interface identifier; the boundary depends on the prefix length. A /64 is the common subnet size for ordinary IPv6 LANs, but it is not a universal rule for every type of link. The block 2001:db8::/32 is reserved for documentation examples, not ordinary public addressing.

Subnetting IPv6 blocks

Subnetting divides an address block into smaller blocks by extending its prefix length. Each bit added to the prefix doubles the number of possible subnets, while the number of addresses in each subnet is halved.

For example, dividing 2001:db8:1234::/48 into /64 subnets extends the prefix by $16$ bits:

This gives $65{,}536$ distinct /64 subnets. The remaining $64$ bits in each /64 block are outside its subnet prefix.

A useful planning habit is to write the prefix length first, then identify which bits are fixed and which are available for subnetting. Unlike IPv4, IPv6 has no broadcast address, and all-zero and all-one values in address fields are generally legal.