3. Why Broadcast?

Broadcasts are useful when a host needs to find information without knowing exactly what other host can supply it, or when a host wants to provide information to a large set of hosts in a timely manner.

When a host needs information that one or more of its neighbors might have, it could have a list of neighbors to ask, or it could poll all of its possible neighbors until one responds. Use of a wired-in list creates obvious network management problems (early binding is inflexible). On the other hand, asking all of one's neighbors is slow if one must generate plausible host addresses, and try them until one works. On the ARPANET, for example, there are roughly 65 thousand plausible host numbers. Most IP implementations have used wired-in lists (for example, addresses of "Prime" gateways.) Fortunately, broadcasting provides a fast and simple way for a host to reach all of its neighbors.

A host might also use a broadcast to provide all of its neighbors with some information; for example, a gateway might announce its presence to other gateways.

One way to view broadcasting is as an imperfect substitute for multicasting, the sending of messages to a subset of the hosts on a network. In practice, broadcasts are usually used where multicasts are what is wanted; packets are broadcast at the hardware level, but filtering software in the receiving hosts gives the effect of multicasting.

For more examples of broadcast applications, see [1, 3].