Ethernet multicasting

When multicasting is implemented at the data link layer, stations are identified by their Media Access Control (MAC) addresses as well as their network level IP addresses.

So far we have seen how multicasting is implemented at the network layer and how multicast IP addresses differ from other types of IP addresses. In addition, we must also distinguish multicasting at the data link layer where stations are identified, not only by their network level IP addresses, but also by their MAC addresses. As opposed to unicast and broadcast addresses, the relation of multicast addressing to IP addressing applies also at this level.

Most local area network (LAN) topologies allow for multicasting by using a group addressing scheme. Some topologies offer better support for multicasting than others. In Ethernet (as defined in IEEE 802.3), all MAC addresses that have the least significant bit of the most significant byte as “1” are multicast addresses. Thus, for example, 01:00:00:00:00:00 and 49:aa:bb:cc:dd:ee are both multicast MAC addresses; while 02:00:00:00:00:00 and fe:fe:fe:fe:fe:fe are not. The devices with a given multicast MAC defined are able to listen to all traffic sent to that particular MAC address.

A specific subset of MAC addresses is reserved for mapping the IP multicasting addresses to data link layer addresses. In Ethernet, the multicast MAC addresses that correspond to multicast IP addresses range from 01:00:5e:00:00:00 to 01:00:5e:7f:ff:ff.