IPv6 Multicast Addresses in Depth

The IPv6 multicast address provides a method for distributing identical information or services to a defined group of interfaces, called the multicast group. Typically, the interfaces of the multicast group are on different nodes. An interface can belong to any number of multicast groups. Packets sent to the multicast address go to all members of the multicast group. For example, one use of multicast addresses is for broadcasting information, similar to the capability of the IPv4 broadcast address.

The following table shows the format of the multicast address.

The following is a summary of the contents of each field.

• 11111111 – Identifies the address as a multicast address.

• FLGS – Set of the four flags 0,0,P,T. The first two flags must be zero. The P field has one of the following values:

  • 0 = Multicast address that is not assigned based on the network prefix

  • 1 = Multicast address that is assigned based on the network prefix

  If P is set to 1, then T must also be 1.

• Reserved - Reserved value of zero.

• Plen - Number of bits in the site prefix that identify the subnet, for a multicast address that is assigned based on a site prefix.

• Group ID - Identifier for the multicast group, either permanent or dynamic.

For complete details about the multicast format, refer to RFC 3306, "Unicast-Prefix-based IPv6 Multicast Addresses.

Some IPv6 multicast addresses are permanently assigned by the Internet Assigned Numbers Authority (IANA). Some examples are the All Nodes Multicast Addresses and All Routers Multicast Addresses that are required by all IPv6 hosts and IPv6 routers. IPv6 multicast addresses can also be dynamically allocated. For more information about the proper use of multicast addresses and groups, see RFC 3307, "Allocation Guidelines for IPv6 Multicast Addresses".