What Is IGMP?

Date First Published: 2nd June 2022

Topic: Computer Networking

Subtopic: Internet Protocols

Article Type: Computer Terms & Definitions

Difficulty Level: 9/10

Learn more about what IGMP is in this article.

Not to be confused with ICMP (Internet Control Message Protocol).

Short for Internet Group Management Protocol, IGMP is a network-layer protocol that is used to establish multicast group memberships and allow devices to join and leave multicast groups, which is when multiple devices receive the same messages at once. It is used to set up multicasting on IPv4 networks, which is when multiple devices receive the same messages. Multicast management on IPv6 networks is handled by Multicast Listener Discovery, which is part of IGMPv6.

Note:

IGMP is not a multicasting routing protocol. It is a protocol used to manage group memberships. In a similar way to ICMP, it does not use a transport-layer protocol, such as TCP or UDP.

How Does IGMP Work?

IGMP works by managing the membership of hosts and routing devices in multicast groups. It only allows the network to direct multicast transmissions to hosts that have requested them.

Hosts connect to a network using IGMP when they want to join a multicast group. Routers that support IGMP listen to IGMP transmissions from hosts to identify which devices belong in which multicast groups. IGMP also uses IP addresses that are set for multicasting, which are in the range of 224.0.0.0 to 239.255.255.255. One of these IP addresses is shared by every multicast group. Whenever a router receives a data packet that is directed at the shared IP address, it will make copies of those data packets and send them to all the members of the multicast group.

Note:

Changes can occur to IGMP multicast groups at any point. It is possible for a host to send an IGMP ‘join group’ or IGMP ‘leave group’ message at any time.

IGMP Messages

IGMP messages are encapsulated in IP datagrams. The IP header of a data packet containing a protocol number of 2 indicates an IGMP packet. The IP data field containing an 8-octet IGMP message indicates an IGMPv2 packet.

Various different types of IGMP messages exist in IGMP, which include:

• Membership report – In order for a device to become a member of a multicast group, devices send these.
• “Leave group” message – These allow devices to leave a multicast group and they are sent from a device to a router.
• General membership queries – A router with multicast capabilities sends these messages to the whole network of devices to update multicast group membership for all groups on the network.
• Group-specific membership queries – These are sent by routers to a specific multicast group rather than the whole network.

IGMP Versions

Three versions of IGMP exist, which include:

• IGMPv1 – The original version of IGMP defined in RFC 1112.
• IGMPv2 – Defined in RFC 2236. This improves IGMPv1 by adding the ability for a host to signal 'desire' to leave a multicast group.
• IMPv3 – Originally defined in RFC 3376. RFC 4604 updates both IGMPv3 and MLDv2 to support source-specific multicast better. It also introduces membership report aggregation.

Note that these versions are backwards compatible.