IPv6 Addressing

IPv6 addresses introduce major changes into the format and method of addressing systems under the Internet Protocol (see RFC 3513 at www.ieft.org/rfc or www.faqs.org for more details). There are several different kinds of addressing with different fields for the network segment. The host segment has been expanded to a 64 bit address, allowing direct addressing for a far larger number of systems. Each address begins with a type field specifying the kind of address, which will then determine how its network segment is organized. These changes are designed not only to expand the address space but to also provide greater control over transmissions at the address level.

IPv6 Address Format

An IPv6 address consists of 128 bits, up from the 32 bits used in IPv4 addresses. The first 64 bits are used for network addressing, of which the first few bits are reserved for indicating the address type. The last 64 bits are used for the interface address, known as the interface identifier field. The number of bits used for sub-netting can be adjusted with a CIDR mask, much like that in IPv4 CIDR addressing (see previous section).

An IPv6 address is written as 8 segments representing 16 bits each (128 bits total). To more easily represent 16 bit binary numbers, hexadecimal numbers are used. Hexadecimal numbers use 16 unique numbers, instead of the 8 used in octal numbering. These are 0 through 9, continuing with the characters A through F.

In the following example, the first four segments represent the network part of the IPv6 address, and the following four segments represent the interface (host) address.

FEC0:0000:0000:0000:0008:0800:200C:417A

You can cut any preceding zeros, but not trailing zeros in any given segment. Segments with all zeros can be reduced to a single zero.

FEC0:0:0:0:8:800:200C:417A

The loopback address used for locahost addressing can be written with seven preceding zeros and a 1.

0:0:0:0:0:0:0:1

Many addresses will have sequences of zeros. IPv6 supports a shorthand symbol for representing a sequence of several zeros in adjacent fields. This consists of a double colon ::. There can be only one use of the :: symbol per address.

FEC0::8:800:200C:417A

The loopback address 0000000000000001 can be reduced to just the following.

::1

To ease the transition from IPv4 addressing to IPv6, a form of addressing incorporating IPv4 addresses is also supported. In this case, the IPv4 address (32 bits) can be used to represent the last two segments of an IPv6 address, and can be written using IPv4 notation.

FEC0::192.168.0.3

IPv6 Interface Identifiers

The identifier part of the IPv6 address takes up the second 64 bits, consisting of 4 segments containing four hexadecimal numbers. The interface ID is an 64 bit (4 segments) Extended Unique Identifier (EUI-64) generated from a network device's Media Access Control (MAC) address.

IPv6 Address Types

There are three basic kinds of IPv6 addresses: unicast, multi-cast, and anycast. These, in turn, can have their own types of addresses.

A unicast address is used for a packet that is sent to a single destination.
A anycast address is used for a packet that can be sent to more than one destination.
A multicast address is used to broadcast a packet to a range of destinations.

In IPv6, addressing is controlled by the format prefix that operates as a kind of address type. The format prefix is the first field of the IP address. The three major kinds of unicast network addresses are global, link-local, and site-local. Global, site-local, and link-local are indicated by their own format prefix (see Table 38-6).

Global addresses begin with the address type 3, site local with FEC, and link-local with FE8. Global addresses can be sent across the Internet.
Link-local addresses are used for physically connected systems on a local network.
Site-local can be used for any hosts on a local network. Site-local addresses operate similarly to IPv4 private addresses, but are used only for local access and cannot be used to transmit over the Internet.

Table 38-6: IPv6 Format Prefixes and Reserved Addresses
IPv6 Addresses Format Prefixes and Reserved Addresses	Description
3	Unicast Global Addresses
FE8	Unicast link-local addresses, used for physically connected hosts on a network
FEC	Unicast Site-local addresses, comparable to IPv4 private addresses
0000000000000001	Unicast Loopback address (for system self-communication, localhost)
0000000000000000	Unspecified address
FF	Multicast Addresses

In addition, IPv6 has two special reserved addresses. The address 0000000000000001 is reserved for the loopback address used for a system's locahost address, and the address 0000000000000000 is the unspecified address.

IPv6 Unicast Global Addresses

IPv6 global address currently uses four fields: the format prefix, a global routing prefix, the sub-net identifier, and the interface identifier. The format prefix for a unicast global address is 3 (3 bits). The global routing prefix references the network address (45 bits), and the sub-net id references a subnet within the site (16 bits).

IPv6 unicast Local Use Addresses: Link-local and Site-local Addresses

For local use, IPv6 provides both link-local and site-local addresses. Link-local addressing is used for interfaces (hosts) that are physically connected to a network. This is usually a small local network. A link-local address uses only three fields: the format prefix FE8 (10 bits), an empty field (54 bits), and the interface identifier (host address) (64 bits). In effect, the network section is empty.

IPv6 site-local addresses have three fields: the format prefix (10 bits), the sub-net identifier (54 bits), and the interface identifier (64 bits). Except for any local subnetting, there is no network address.

IPv6 Multicast Addresses

Multicast addresses have a format prefix of FF (8 bits) with flag and scope fields to indicate whether the multicast group is permanent or temporary and whether it is local or global in scope. A group identifier (112 bits) references the multicast group. For the scope, 2 is link-local, 5 is site-local, and E is global. In addition to their interface identifiers, hosts will also have a group id that can be used as a broadcast address. You use this address to broadcast to the hosts. The following example will broadcast only to those hosts on the local network (5) when the group id 101.

FF05:0:0:0:0:0:0:101

To broadcast to all the hosts in a link-local scope you would use the broadcast address:

FF02:0:0:0:0:0:0:1

For a site-local scope, a local network, you would use:

FF05:0:0:0:0:0:0:2