Say you are at a party and you need to talk to a woman named Suzy.
Unicast DNS is like asking the host of the party who she is;
multicast DNS is like shouting “Is Suzy here?” to the whole room.
Naturally, things would get awfully loud if it was a big party, but Zeroconf was designed for small local networks, so it isn’t a problem. And best of all, no one person has to know everyone in the room — being a host is tough, and it’s the user who usually ends up with the job.
Service discovery uses the same broadcast mechanism, but instead of looking for a particular person, you look for capabilities.
It’s like yelling out, “Anyone here know how to mix a Tequila Sunrise?” and waiting to see who puts their hand up.
– From Understanding Zeroconf and Multicast DNS by Heath Johns