EMZETT.Multicast
In short: A transmission from one sender to a defined group of recipients who have registered for this group — neither to everyone (broadcast) nor to just one (unicast).
In more detail: Recipients register with a multicast group via IGMP, and routers then forward the traffic specifically only there. Efficient for applications with many simultaneous recipients of the same data stream, e.g. IPTV or stock price feeds.
In Depth
The decisive efficiency advantage of multicast over many individual unicast connections shows in routing: instead of the sender sending the same data separately once per recipient (which burdens the sender’s bandwidth and every network link along the way several times over), it sends the data exactly ONCE to a special multicast destination address. Routers along the way only multiply the data stream where the paths to the individual recipients actually branch — so every data packet flows only once over every section of the route, no matter how many recipients are listening in total.
In IPv4, the addresses 224.0.0.0 to 239.255.255.255 are reserved for multicast. Recipients interested in a particular multicast group register this with their local router via IGMP (Internet Group Management Protocol) — the router remembers these registrations and forwards incoming packets for this group accordingly. Typical areas of use are live video streaming to many simultaneous viewers, router announcement protocols and financial data feeds, where the same data has to go to a very large number of subscribers in real time.