The announcement information is multicast to a well-known IP address and port where clients running the session directory tool receive this information. In addition to SDP, there are other ways that multicasts can be announced. For example, on the corporate intranet, multicasts can be advertised using web pages. Controls embedded in the web page can then receive the multicast data. Joining Groups To signal that they want to receive a multicast, clients join the group to whom the multicast is directed.
Multicast groups provide several advantages.
No elaborate scheme is required to create or disband a group. When a group has no members, it ceases to exist on the network. Groups also scale upward easily because as more clients join a multicast, it becomes more likely that the multicast is already being routed close to them. Second, the client sets its IP process and network card to receive the multicast on the group's address and port. Multicast addresses are Class D IP addresses ranging from 224.0.0.0 to 239.255.255.255.