Multicast addresses

Multicast addresses provide one-to-many communication.

An IPv6 multicast address identifies a group of nodes.

The scope is built into the multicast address structure.

The system uses a multicast address to send traffic to multiple destinations. In this situation traffic experiences less delay with a multicast address than it would with Unicast address.

The following figure shows the format of an IPv6 multicast address.

IPv6 multicast address format

A value of FF (11111111) in the 8 high-order bits of an IPv6 address indicates that the address is an IP multicast address.

The Multicast IPv6 Prefix is FF00::/8 (RFC3513).

Flags

The 4-bit flags field indicates whether the group is permanent or transient. The first 3 bits are reserved and the 4th bit represents the Transient flag. Currently only the Transient (T) flag is defined. A T flag set to 0 specifies a permanently assigned multicast address. A T flag set to 1 specifies a transient address.

Group ID

The 112 bit group ID identifies the multicast group.

An example of a multicast address is FF01:0:0:0:0:0:0:101

Scope field

The 4-bit scope field within the group ID specifies the multicast traffic scope.

Examples of multicast addresses

All-nodes addresses look like this:

FF01::1 (Node Local), FF02::1 (Link Local)

All-routers addresses look like this:

FF01::2 (Node Local), FF02::2 (Link Local)

A solicited node or host address looks like this:

FF02::1:FF1E:8329.

In this case the MAC is 00-02-B3-1E-83-29 and the IPv6 address is fe80::202:B3FF:FE1E:8329.

The following table lists some well-known multicast IPv6 addresses