The Internet Group Management Protocol (IGMP) is a communications protocol used to manage the membership of Internet Protocol multicast groups. IGMP is used by IP hosts and adjacent multicast routers to establish multicast group memberships.
It is an integral part of the IP multicast specification, operating above the network layer, though it doesn't actually act as a transport protocol. It is analogous to ICMP for unicast connections. IGMP can be used for online streaming video and gaming, and allows more efficient use of resources when supporting these types of applications. IGMP does allow some attacks, and Firewall and types commonly allow the user to disable it if not needed.