Introduction to Network Channels, Processors, and Media

   •In packet switching, the output links to which a packet should be forwarded is not known before their arrivals. Unlike in time-division multiplexing, there is no telling exactly how many packets from an input will be destined for each output over a given period of time.

A router is like a bridge, but with a key difference. Bridges are Data Link layer devices, so they can tell the MAC address of the network node to which each message is sent, and can forward the message to the appropriate segment. However, they can’t peek into the message itself to see what type of information is being sent. In contrast, a router is a Network layer device, so it can work with the network packets at a higher level. In particular, a router can examine the IP address of the packets that pass through it. And because IP addresses have both a network and a host address, a router can determine what network a message is coming from and going to. Bridges are ignorant of this information.