There are 3 basic methods of communicating over an Ethernet IP network

Unicast –a one to one communication
Broadcast – a one to all communication
Multicast – a one to several communication

Multicasting is a method of saving network bandwidth when multiple receivers wish to receive identical data. If for instance an IP camera is producing large amounts of network traffic that is required to be seen on a number of monitors it would be very inefficient and may cause network delays if a unicast transmission were used.

A protocol call IGMP (Internet Group Management Protocol) is used.

A data producer will send data in a UDP (User Datagram Protocol) packet to a multicast IP address in the range 224.0.0.0 to 239.255.255.255. Each of these addresses is referred to as a group. Multiple groups can exist on the same network.
UDP is a fire and forget mechanism so there is no guaranteed delivery protocol on a multicast packet.

Multicasting data is used in video over IP, certain industrial Ethernet protocols like Ethernet/IP and is also useful when multidrop serial protocols are embedded into Ethernet packets.

The Problem with Multicasting

Multicast is a very helpful way to reduce traffic on a network however by its nature on a basic Ethernet switched network all the multicast data traffic is transmitted to all points of the network – effectively voiding one of the main benefits of the Ethernet switch which is to manage the bandwidth of the network by only transmitting data to the points where it is required.

When multicast traffic is used in a routed network the IGMP protocol is used to restrict data to only parts of the network where it is required. This is handled by receivers sending a join message to the router effectively switching on the data stream. If the data is no longer required a leave message is sent and the router stops sending the multicast data down to that part of the network. This way the multicast traffic will only be seen on parts of the network where it is required.

IGMP is a level 3 protocol effectively meant for routers however it can also be used on advanced switched networks to control the dataflow. Ethernet switches are designed to make their switching decision on the layer 2 data (the Ethernet addresses), however by snooping into the layer 3 (IP address) data it is possible for switches to also control the flow of multicasting around a network. The technique is often referred to as IGMP snooping.

Using switches with IGMP snooping is important when there is a lot of multicast data on a network.