Three Delivery Models
A packet can be aimed at exactly one receiver, at a group of interested receivers, or at everyone on a segment. These are unicast, multicast, and broadcast.
Unicast
Unicast is one to one delivery to a single destination address. It is by far the most common model and underlies normal web, email, and API traffic. Each receiver needs its own copy, so sending the same data to many hosts means many separate streams.
Multicast
Multicast is one to many delivery to a group of receivers that have opted in. The sender transmits once and the network duplicates packets only where paths branch toward interested receivers.
- Receivers join a multicast group to start receiving.
- It is efficient for streaming the same content to many listeners.
- It works best on managed networks rather than across the open internet.
Broadcast
Broadcast is one to all delivery to every host on the local segment. It is used for discovery tasks like DHCP and address resolution, but it does not cross routers, which keeps broadcasts contained to a single network. IPv6 drops broadcast entirely in favor of multicast.
Key idea
Unicast targets one host, multicast targets an opted in group with the network duplicating only where needed, and broadcast reaches every host on the local segment.