What Is Multicast?

Multicast is a technology used to address multiple hosts as a group. A host multicasts to a group of hosts by sending an IP packet to a special IP address that identifies the group. An IP address is used to designate a group of hosts. The IP address that defines a multicast group is in the range of 224.0.0.0 to 239.255.255.255, defined as Class D addresses. Each group has its specific IP address in that range, so many groups can be defined at the same time by using a different IP address.

The membership in a group is dynamic because hosts can join and leave the group as they want. The multicast standard is described in RFC1112.

Multicast is similar to broadcast since only one packet is sent to all hosts, whereas unicast means that one packet is sent to one host. However, multicast differs from broadcast because broadcasting involves sending a packet to all hosts, without exception, on the specified network, whereas multicast sends a packet to the group of hosts. Those hosts that are not part of the group will not process the multicast packet since it is not addressed to them.

One typical example of multicasting is to listen to a videoconference using a 2 Mbits/sec channel on a...