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| 7.2 |  | Examining Multicast in a Multilayer Switched Network |
| 7.2.2 |  | Multicast addressing |

Whenever an application needs to send data to more than one station, but wants to restrict the distribution to only 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 stations in the network. 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. 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 configuring 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.