IGMP

The Internet Group Management Protocol (IGMP) is another little-seen but important Transport layer protocol. The challenge to multicasting is determining who wants to receive the multicast and who does not. IGMP is the glue that routers use to communicate with hosts to determine a "group" membership. The multicast Class D licenses are assigned the network ID of 224/8. However, multicast doesn't assign IP addresses to individual hosts in the same manner as you've seen thus far. Instead, a particular multicast (called an IGMP group) is assigned to a certain 224/8 address and those who wish to receive this multicast must tell their upstream router or switch (which must be configured to handle multicasts) that they wish to receive it.

To do so, they join the IGMP group and then the router or switch knows to send that multicast data stream to that client.

Tip There is some argument over where ICMP and IGMP fit in the OSI seven-layer model. Both of these protocols lack port numbers, so for many people that makes ICMP and IGMP fit more into the Network layer than the Transport layer. When in doubt, place TCP and UDP in the Transport layer and ICMP and IGMP in the Network layer.