Multicast Definition

A multicast is a single stream of data (i.e., a set of packets) that is transmitted simultaneously to selected multiple hosts who have joined the appropriate multicast group.

In contrast to broadcasts (which are used on some LANs), multicast clients receive the data stream only if they have previously elected to do so (i.e., by joining the specific multicast group address). A unicast is a transmission of a distinct stream to a single client.

Multicasting is analogous to radio and television broadcasts in the sense that only those who have tuned their receivers (by selecting a particular frequency they are interested on) receive the information.

Multicasting is different from traditional streaming media, which is a type of unicasting. The reason is that the multicasting server only has to generate a single data stream, which can be received by all recipients, rather than a separate one for each recipient.Multicasting is much more efficient than unicasting with regard to bandwidth conservation.

Multicasting is suitable for the situation in which there is a large amount of information to be transmitted to various (but usually not all) hosts on a network. It is commonly used when distributing real time audio and video to the set of hosts which have joined a distributed conference. It is also used by video servers for sending out networked television channels.

Unicast transmissions appeared to be sufficient for the Internet for a number of years. However, when multicasting was implemented in 1993 in the 4.4 BSD release, it immediately became clear that it had great potential.

The IP addresses through are reserved for multicasts. These multicast addresses are also known as Class-D IP addresses.

Created October 20, 2005.
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