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.