Broadcast and Multicast

Most of this discussion of IP traffic has revolved around the process of unicast traffic, which is traffic that is addressed for a single host. However, IP traffic can also be broadcast or multicast traffic, providing for some flexibility in how traffic is delivered.

Broadcast traffic is traffic that is destined to all hosts on a given subnet or to all hosts on all subnets. Broadcasts take advantage of the fact that the electrical signal is actually received by all hosts unless otherwise prevented. If a host needs to send the same data to multiple hosts, instead of needing to repeat the data in a unicast fashion to all destinations, it can just broadcast the data one time, allowing all the hosts to receive the data. For this reason, broadcast traffic is often referred to as "one-to-all" traffic. The drawback of broadcasts is that hosts that do not necessarily need to process the data will still receive the data because it is broadcast to every host on a network.

Multicasts attempt to bridge the gap between a unicast and a broadcast by functioning much like a broadcast, but limiting the destination hosts to only those which specifically register to receive the multicast. This allows a host to send the data a single time, but only the hosts that have specifically registered to receive that particular traffic will process it. Consequently, this is known as "one-to-many" or "one-to-some" traffic. Multicast traffic is typically used for applications that stream data, such as audio or video. It is also frequently used by routers to locate and pass routing protocol information between each other.