Multicast is when, on a computer network, one device sends a stream of packets to a group of devices, which starts out as a single stream which is then duplicated by the routers through which it passes as necessary.
It is an alternative to Unicast where a computer sends data to one other device only.
If you want to send the same stream to multiple devices, for instance for broadcasting a TV or radio channel, it is much more efficient compared to multiple unicast streams. But it does not allow a receiving device to stop or rewind the source and it prevents the sender from easily knowing how many devices are receiving the stream.
In a broadcast center, it is commonly used for IP ring mains. Broadcast audio and video over IP mostly uses IP multicast, and with the appropriate control software can duplicate the functionality of an XY Broadcast Router (Matrix, NOT a IT network router). When switching a route, what is happening under the lid is that a receiving device joins a multicast stream. Challenges include how to cleanly switch video because the video timing (position of the vertical blanking interval) is invisible to the IT networking layers. One option is for the receiving device to join the new stream before leaving the old one, and to switch the decoded video.
