multicast
termnetworking
One-to-many delivery: a single transmission sent to a group address reaches every host that joined the group, and only them.
Steve Deering defined IP multicast in RFC 1112 (August 1989): senders transmit once to a group address, the network replicates only where receivers exist, and hosts join or leave groups with IGMP (MLD in IPv6). IPv4 reserves the old class D space, 224.0.0.0/4, for it; IPv6 uses ff00::/8. Routing protocols lean on it constantly, OSPF speaks to 224.0.0.5 and 224.0.0.6, and it carries IPTV, market data feeds, and service discovery. The efficiency is the point: one stream serves a thousand receivers without unicasting a thousand copies or broadcasting to hosts that never asked.
Multicast sends one copy of a stream that the network duplicates only where paths diverge, so a thousand receivers cost roughly what one costs at the source. For anything genuinely one-to-many and simultaneous, the arithmetic is unbeatable, which is why it carries broadcast video, financial market data, and service discovery.
The reason it never became ubiquitous is that it demands the network participate. Routers must run a multicast routing protocol, switches must snoop group membership or they flood every port, and every device in the path must agree. That is achievable inside one administrative domain and extremely difficult across the public internet, which is why internet-scale video streaming is unicast and content delivery networks exist to make that affordable.
Where it does live is worth knowing. Local discovery protocols use it constantly, which is how a laptop finds a printer without configuration, and IPv6 replaced broadcast with multicast entirely, so neighbour discovery is multicast rather than shouting at everyone. On a switch without proper snooping, that traffic reaches every port, which is a quiet performance drain that looks like nothing in particular.