IGMP Messages

Before a host can receive and process multicast datagrams, it has to do two things:
it must inform the local network interface card that it wishes to accept and process
network frames for specific multicast groups, and it must inform any multicast
routers on the local network that it wishes to receive the IP packets for those
multicast groups as well.
How the application communicates with the local network interface card is a function
of the specific IP implementation in use on the local system (the process is discussed
earlier in this chapter in “Local Multicasting”). However, the mechanisms
used by IP to inform the multicast routers on the local network of its desire to participate
in certain multicast groups is a function of the Internet Group Management
Protocol (this process is discussed earlier in “Managing Group Memberships”).