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. In this way 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.
One key difference between a bridge and a router is that a bridge is essentially transparent to the network. In contrast, a router is itself a node on the network, with its own MAC and IP addresses. This means that messages can be directed to a router, which can then examine the contents of the message to determine how it should handle the message.
You can configure a network with several routers that can work cooperatively together. For example, some routers are able to monitor the network to determine the most efficient path for sending a message to its ultimate destination.
If a part of the network is extremely busy, a router can automatically route messages along a less-busy route. In this respect, the router is kind of like a traffic reporter up in a helicopter. The router knows that the 101 is bumper-to-bumper all the way through Sunnyvale, so it sends the message on 280 instead.
Here's some additional information about routers:
The functional distinctions between bridges and routers — and switches and hubs, for that matter — get blurrier all the time. As bridges, hubs, and switches become more sophisticated, they’re able to take on some of the chores that used to require a router, thus putting many routers out of work.
Some routers are nothing more than computers with several network interface cards and special software to perform the router functions.
Routers can also connect networks that are geographically distant from each other via a phone line (using modems) or ISDN.
You can also use a router to join your LAN to the Internet. The illustration below shows a router used for this purpose.
