IPv6 Address Types
There are three major categories of IPv6 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.
A unicast address identifies a single interface. When a network device sends a packet to a unicast address, the packet goes only to the specific interface identified by that address. Unicast addresses support a global address scope and two types of local address scopes.
A unicast address consists of n bits for the prefix, and 128 – n bits for the interface ID.
In the IPv6 implementation for a subscriber access network, the following types of unicast addresses can be used:
Global unicast address—A unique IPv6 address assigned to a host interface. These addresses have a global scope and essentially the same purposes as IPv4 public addresses. Global unicast addresses are routable on the Internet.
Link-local IPv6 address—An IPv6 address that allows communication between neighboring hosts that reside on the same link. Link-local addresses have a local scope, and cannot be used outside the link. They always have the prefix FE80::/10.
Loopback IPv6 address—An IPv6 address used on a loopback interfaces. The IPv6 loopback address is 0:0:0:0:0:0:0:1, which can be notated as ::1/128.
Unspecified address—An IPv6 unspecified address is 0:0:0:0:0:0:0:0, which can be notated as ::/128.
A multicast address identifies a set of interfaces that typically belong to different nodes. When a network device sends a packet to a multicast address, the device broadcasts the packet to all interfaces identified by that address. IPv6 does not support broadcast addresses, but instead uses multicast addresses in this role.
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.
The following types of multicast addresses can be used in an IPv6 subscriber access network:
Solicited-node multicast address—Neighbor Solicitation (NS) messages are sent to this address.
All-nodes multicast address—Router Advertisement (RA) messages are sent to this address.
All-routers multicast address—Router Solicitation (RS) messages are sent to this address.
Multicast addresses use the prefix FF00::/8.
An anycast address identifies a set of interfaces that typically belong to different nodes. Anycast addresses are similar to multicast addresses, except that packets are sent only to one interface, not to all interfaces. The routing protocol used in the network usually determines which interface is physically closest within the set of anycast addresses and routes the packet along the shortest path to its destination.
There is no difference between anycast addresses and unicast addresses except for the subnet-router address. For an anycast subnet-router address, the low-order bits, typically 64 or more, are zero. Anycast addresses are taken from the unicast address space.
For more information about anycast addresses, see RFC 2526, Reserved IPv6 Subnet Anycast Addresses.