9.1 Multicasting Overview
9.1.2 Multicast addressing
Whenever an application needs to send data to more than one station, but only to stations interested in receiving the traffic, the application typically uses a multicast destination address. Whereas a broadcast would target all users in a domain, multicast addresses target a select group of all stations in the network. These are both illustrated in Figures and . On the other hand, if the source transmits unicast frames, it would have to send multiple copies of the frame, one addressed to each intended receiver. As you can see in Figures and , this is a very inefficient use of network resources and does not scale well as the number of receivers increases.

By using multicast addresses, the source transmits only one copy of the frame into the network, and routers distribute the multicast message to the other segments where interested receivers reside. As discussed previously, multicast addresses appear at Layers 2 and 3. A network administrator assigns the multicast Layer 3 address for an application. The Layer 2 multicast address is then calculated from the Layer 3 multicast address. When you configure a multicast application, the NIC adds the multicast address to its list of valid Media Access Control (MAC) addresses. Usually this list consists of the built-in MAC address plus any user-configured addresses. Whenever the station receives a frame with a matching multicast destination address, the receiver sends the frame to the CPU.

The router examines multicast addresses at both Layers 2 and 3, whereas a switch examines only the Layer 2 address. However, some high-end switches with Supervisor III cards and NFFCs can examine the Layer 3 addresses as well. Otherwise, the switch simply examines the MAC address in the frame.