Internet Core Protocols: The Definitive Guide

An Introduction to IP Multicasting

In essence, IP multicasting is a function of the IP address in use by a particular “multicast group.” If a user wants to send an audio presentation to a variety of distributed hosts simultaneously, then that user would send the data to an IP address that was associated with a particular multicast group. Any hosts that were listening for traffic destined for that IP address would then receive the data and process it accordingly, while other hosts would simply ignore it.

The application protocol used for such a feed could be a RealAudio stream sent as a bunch of UDP messages over IP. These messages would not look any different from the same data being sent to a single destination system or to the local broadcast address. The only difference between the multicast datagrams and their unicast or broadcast equivalents would be the destination IP address in use with those datagrams: the unicasts would point to a specific destination system, the broadcasts would point to all hosts on the local network, and the multicasts would point to a group address.

In the IP world, multicast group addresses are known as Class D addresses and include all of the IP addresses in the range of 224.0.0.0 through 239.255.255.255. Each of the individual addresses in this range refer to specific multicast groups, many of which are associated with a specific application or service.

There are a number of predefined, reserved addresses that are registered with the Internet Assigned ...