Internet Group Management Protocol (IGMP)
IGMP is the protocol that devices on IPv4 networks use to tell routers which multicast groups they want to receive.
What is Internet Group Management Protocol (IGMP)?
When a set-top box wants an IPTV channel sent by multicast, it sends an IGMP "join" for that group, and routers then forward the stream only where someone has asked for it. IGMP snooping is a feature of switches and home gateways: they listen to these messages so that multicast traffic goes only to the ports that requested it.