Unicast, broadcast, and multicast
The three types
- Unicast, one sender to one specific destination: the predominant form on LANs and the internet.
- Broadcast, one to all addresses: the same message to every device on the LAN.
- Multicast, one to a specific group: receivers must be members of the multicast group to get the traffic.
For broadcast and multicast, the source still sends only one frame. The switch performs replication and regeneration, copying it to the ports that should receive it: all ports for broadcast, group-member ports for multicast.
How multicast delivery works
The group is represented by a specific multicast IP address (like 239.1.1.1) with a corresponding multicast MAC. Receivers send Join messages declaring they want traffic for that multicast IP.
The source just sends; only hosts listening to that group address accept the packets.
Two protocols build the path:
- IGMP (Internet Group Management Protocol): works inside the local LAN. It handles the Join messages from receivers and makes sure multicast traffic only goes to group members.
- PIM (Protocol-Independent Multicast): a multicast routing protocol. It builds the multicast distribution trees so packets from the source reach every joined receiver across routers.
The mental split: IGMP is hosts telling the LAN “I want in.” PIM is routers building the delivery tree between networks.