What's the difference between broadcast and multicast?

Question

I'm sending a packet indiscriminately to other clients on my network. Is this broadcast or multicast?

If you're in a Jabber chat room and you send a message to the room, is this broadcasting or multicasting?

Generally, how can you tell which is which?

Solution 1

Broadcast packets are sent to all computers on your subnet. Multicast packets work similar, but can cross routers. How far they go depends on how the actual network is configured. In the home they never cross the router to get to the internet unless something really special has been done. In the corporate landscape, it largely depends on what multicast protocols are needed. HP printers have been known to issue multicast service-location-protocol packets, which is one example of them. Some networks turn it off completely, others don't. It depends.

Jabber the protocol doesn't have any broadcast or multicast abilities. When you send a message to everyone in the room what's happening is directed unicast. Typically the Jabber server will take your input and then send it to each connected computer individually. However, the term Multicast is used in some Jabber docs, but it doesn't hold the same meaning as broadcast/multicast does in my first paragraph.

Solution 2

Translating this to the real world, broadcasting would be the free newspaper delivered to all adresses in an area, multicast would be adressed envelopes all holding the same content but sent to specific recipients...

Solution 3

In general terms, "broadcast" is sending the same information to all clients, and whoever is interested can listen in. "Multicast" is sending the same information to multiple listening clients in a single operation.

In TCP/IP terms, "broadcast" is sending the same packets to the entire local CIDR network broadcast address. "Multicast" is sending packets to a multicast address.