multicast routing
Imagine a live lecture being sent to a hundred classrooms. Mailing a separate copy to each room (unicast) wastes effort; shouting it to literally everyone in the building (broadcast) annoys those who did not ask. The smart middle path is to send one stream that the network duplicates only where it must, so each interested classroom gets a copy without the source sending a hundred. Multicast routing builds the delivery tree that makes this efficient one-to-many delivery possible.
Multicast uses special group addresses (in IPv4, the 224.0.0.0/4 range): a packet sent to a group address should reach every host that has joined that group, and no others. The job of multicast routing is to compute a distribution tree rooted at the source (or shared among sources) whose branches reach exactly the routers with interested receivers, so each link carries the stream at most once and routers fork copies only at branch points. Receivers signal interest by joining a group (via a protocol like IGMP to their local router), and routers build the tree with multicast routing protocols such as PIM. Closely related is broadcast routing, which floods to all hosts in a network — multicast is the selective, group-aware version.
It matters wherever the same data goes to many receivers at once: IPTV and live video, software and stock-quote distribution, and routing protocols themselves (OSPF uses multicast to reach neighbors). Done right, multicast sends each byte over each link just once regardless of how many receivers hang off it downstream — a huge saving over sending a separate unicast copy per receiver.
An honest caveat: IP multicast works well within a single managed network but is famously hard to deploy across the wider Internet, because it needs every router along the path to cooperate, maintain per-group state, and trust group membership across administrative boundaries. As a result, much 'one-to-many' delivery on today's Internet is actually done with unicast plus content delivery networks rather than true network-layer multicast.
A source streams video to multicast group 239.1.1.1. Three of a router's five downstream links lead toward joined receivers, so the router replicates the stream onto exactly those three links and not the other two — one inbound copy becomes three, only where needed.
Routers fork copies only at branch points of the distribution tree, so each link carries the stream at most once.
True network-layer IP multicast is rarely deployed across the public Internet; most large-scale one-to-many delivery today uses unicast plus CDNs instead.