/ˌaɪ dʒiː ɛm piː/
noun — "managing who joins and leaves network multicast groups."
IGMP (Internet Group Management Protocol) is a communications protocol used in IPv4 networks to manage membership in multicast groups. Multicast allows a single packet stream to be delivered efficiently to multiple recipients without sending separate copies to each host. IGMP enables hosts to report their interest in joining or leaving multicast groups to neighboring routers, which then control the distribution of multicast traffic across the network.
Technically, IGMP operates between hosts and routers on a local network segment. Hosts send IGMP Membership Reports to indicate they want to receive traffic for a specific multicast address. Routers periodically issue IGMP Queries to verify active memberships. The protocol supports multiple versions (IGMPv1, IGMPv2, IGMPv3), each introducing enhancements such as leave messages and source-specific multicast filtering. By maintaining accurate group membership tables, IGMP minimizes unnecessary network traffic and ensures that multicast streams are only forwarded where needed.
Key characteristics of IGMP include:
- Multicast membership management: tracks which hosts want specific multicast streams.
- Versioned operation: IGMPv1, v2, and v3 provide increasing functionality for leave reporting and source filtering.
- Router coordination: ensures multicast traffic is delivered only to networks with interested hosts.
- Efficiency: reduces bandwidth usage compared to multiple unicast streams.
- IPv4 focus: specifically designed for IPv4; IPv6 uses the MLD protocol.
In practical workflows, IGMP is fundamental in streaming video, IPTV, and enterprise multicast applications. For example, when multiple users subscribe to a live video feed on a corporate network, their devices send IGMP reports to indicate interest. The network router forwards the multicast packets only to segments where members exist. If a device leaves the group, IGMP leave messages or query responses allow the router to stop forwarding traffic to that segment, conserving bandwidth.
Conceptually, IGMP acts like a club registrar: it keeps track of who wants to attend a group event (receive a multicast stream) and informs the organizers (routers) so resources are allocated efficiently, without sending invitations to uninterested parties.
Intuition anchor: IGMP enables networks to deliver data collectively, making multicast communication efficient, scalable, and responsive to dynamic membership.