Multicast

On the same local network, a phone can see a TV, a computer can see a printer, and a speaker can be used for casting, even when there is no dedicated DNS server doing local name resolution for them. When a device first joins the network, it may not even know who it is supposed to ask. Still, it has to recognize names first so that service discovery has somewhere to land.

When the same data has to go to many receivers, there are usually only two extremes: send a separate copy to each one, or broadcast it to the whole network. The first is simple, but once the number of receivers grows, the sender and the network get buried under duplicate traffic. The second looks convenient, but it also disturbs devices that should never have received that data in the first place. Live video, market data distribution, service discovery, routing neighbor announcements, and many local-link control messages all need something different from either extreme.