Understanding IPv6 Addressing

IPv6 Address Representation

IPv6 addresses consist of 8 groups of 16-bit hexadecimal values separated by colons (:). IPv6 addresses have the following format:

 aaaa:aaaa:aaaa:aaaa:aaaa:aaaa:aaaa:aaaa

Each aaaa is a 16-bit hexadecimal value, and each a is a 4-bit hexadecimal value. Following is a sample IPv6 address:

 3FFE:0000:0000:0001:0200:F8FF:FE75:50DF

You can omit the leading zeros of each 16-bit group, as follows:

 3FFE:0:0:1:200:F8FF:FE75:50DF

You can compress 16-bit groups of zeros to double colons (::) as shown in the following example, but only once per address:

3FFE::1:200:F8FF:FE75:50DF

An IPv6 address prefix is a combination of an IPv6 prefix (address) and a prefix length. The prefix takes the form ipv6-prefix/prefix-length and represents a block of address space (or a network). The ipv6-prefix variable follows general IPv6 addressing rules. The /prefix-length variable is a decimal value that indicates the number of contiguous, higher-order bits of the address that make up the network portion of the address. For example, 10FA:6604:8136:6502::/64 is a possible IPv6 prefix.

For more information on the text representation of IPv6 addresses and address prefixes, see RFC 4291, IP Version 6 Addressing Architecture.

IPv6 Address Types

IPv6 has three types of addresses:

• Unicast—For a single interface.
• Multicast—For a set of interfaces on the same physical medium. A packet is sent to all interfaces associated with the address.
• Anycast—For a set of interfaces on different physical media. A packet is sent to only one of the interfaces associated with this address, not to all the interfaces.

IPv6 Address Scope

Unicast and multicast IPv6 addresses support address scoping, which identifies the application suitable for the address.

Unicast addresses support global address scope and two types of local address scope:

• Link-local unicast addresses—Used only on a single network link. The first 10 bits of the prefix identify the address as a link-local address. Link-local addresses cannot be used outside the link.
• Site-local unicast addresses—Used only within a site or intranet. A site consists of multiple network links. Site-local addresses identify nodes inside the intranet and cannot be used outside the site.

Multicast addresses support 16 different types of address scope, including node, link, site, organization, and global scope. A 4-bit field in the prefix identifies the address scope.

IPv6 Address Structure

Unicast addresses identify a single interface. Each unicast address consists of n bits for the prefix, and 128 – n bits for the interface ID.

Multicast addresses identify a set of interfaces. Each multicast address consists of the first 8 bits of all 1s, a 4-bit flags field, a 4-bit scope field, and a 112-bit group ID:

11111111 | flgs | scop | group ID

The first octet of 1s identifies the address as a multicast address. The flags field identifies whether the multicast address is a well-known address or a transient multicast address. The scope field identifies the scope of the multicast address. The 112-bit group ID identifies the multicast group.

Similar to multicast addresses, anycast addresses identify a set of interfaces. However, packets are sent to only one of the interfaces, not to all interfaces. Anycast addresses are allocated from the normal unicast address space and cannot be distinguished from a unicast address in format. Therefore, each member of an anycast group must be configured to recognize certain addresses as anycast addresses.