System Administration Guide: IP Services

Multicast Addresses

IPv6 supports the use of multicast addresses. The multicast address identifies a multicast group, which is a group of interfaces, usually on different nodes. An interface can belong to any number of multicast groups. If the first 16 bits of an IPv6 address is ff00n, the address is a multicast address.

Multicast addresses are used for sending information or services to all interfaces that are defined as members of the multicast group. For example, one use of multicast addresses is to communicate with all IPv6 nodes on the local link.

When an interface's IPv6 unicast address is created, the kernel automatically makes the interface a member of certain multicast groups. For example, the kernel makes each node a member of the Solicited Node multicast group, which is used by the Neighbor Discovery protocol to detect reachability. The kernel also automatically makes a node a member of the All-Nodes or All Routers multicast groups.

For detailed information about multicast addresses, refer to IPv6 Multicast Addresses in Depth. For technical information, see RFC 3306, Unicast-Prefix-based IPv6 Multicast Addresses, which explains the multicast address format. For more information about the proper use of multicast addresses and groups, RFC 3307, Allocation Guidelines for IPv6 Multicast Addresses.