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The Internet Group Management Protocol (IGMP) is used to provide information about membership of multicast groups on a network. Multicasting allows the efficient distribution of information to many hosts without requiring that inefficient alternatives such as multiple host-to-host transmissions, and broadcasting be used. Multicasting also allows clients to locate servers without broadcasting by requesting membership of the multicast group that the server maintains.
Hosts that wish to join a multicast group instruct their network adapter cards to listen for network frames specifying a special MAC address that corresponds to the multicast class D IPv4 address of the group. (In practice, the mapping is not unique, so the IPv4 module also has to perform some filtering of received frames.)