IGMP

protocol that allows Internet servers to transmit a multicast of messages to a group of computers

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, like ICMP for unicast connections. IGMP can be used for online video and gaming, and allows more efficient use of resources when supporting these uses. IGMP does allow some attacks , and firewalls commonly allow the user to disable it if it will not be needed.
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Internet Group Management Protocol

INTERNET GROUP MULTICAST PROTOCOL. IGMP