This chapter deals with a few other topics that are too important to leave out, but didn't readily fit into other parts of this book: IP multicast, IPv6, and security. Not every site needs to employ these topics initially. To varying extents, they can all be retrofitted into existing networks.
Most TCP/IP applications operate like a telephone conversation. That is, one device makes a connection with another, they exchange information, and then they disconnect. This activity is appropriate and efficient for some types of applications. Allowing any device to call up any other device avoids the overhead of maintaining a mesh network in which every device is permanently attached to every other.
There are some types of applications that do not work well in this telephone-call model, though. For example, it would be extremely inefficient to run a radio station this way. Radio stations work by broadcasting a single signal. This common signal is received by many end devices simultaneously. Thus, everybody who listens to this station hears the same news or music at the same time. It would be extremely inefficient if this simultaneous broadcast required sending the same signal separately to every device.
Sending the same signal thousands of times is not only inefficient on the server; it also uses the network bandwidth poorly. Radio and television broadcasting are effective partly because the signals are sent only once. Sending the signals ...