Multicast (MultiCast) is a point-to-multipoint (or multipoint-to-multipoint) communication method, that is, multiple receivers receive the same information sent by a source at the same time. In multicast, the concept of group is very important. According to the definition of multicast, a multicast message is sent from one source to three groups of destination nodes. In IP multicast, the multicast group has an ID number, called the multicast group ID number, which specifies the destination group to send the message, which is actually a class D IP address. If a host wants to receive multicast messages sent to a specific group, it must monitor all messages sent to that specific group.
In order to resolve the routing of multicast messages on the Internet, the host must notify the multicast router on its subnet to join a group, and the Internet Group Management Protocol (IGMP) is used in multicast to achieve this goal.
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