Multicast

Multicast is a one-to-many communication and can be very efficient. For multicast, we use the space FF00::/8 and an interface may belong to any number of multicast groups; this does appear to work well in IPv6. Multicast replaces broadcasts, there are no broadcasts in IPv6.

The first two bytes (FF) define multicast, the next byte can be a 0 for a permanent or well-known address. The flag is 1 for a temporary address.

The fourth byte is the scope, where;

1 = for the scope of the interface (loopback transmission)

2 = for the link scope (similar to unicast link-local scope)

3 = for subnet-local scope where subnets may span multiple links

4 = for admin-local scope (administratively configured)

5 = for the site scope

8 = for the organizational scope (multiple sites)

E = for the global scope

For example, a multicast address starting with FF02::/16 is a permanent multicast address with a link-local scope. There are some reserved link-local multicast addresses, as there are in IPv4.

• FF02::1 — All nodes on link

• FF02::2 — All routers on link

• FF02::5 — All OSPF routers on link

• FF02::6 — All Designated Routers on link

• FF02::9 — All RIP routers on link