Bridges

A bridge is a device that connects two networks so that they act as if they are one network. Bridges are used to partition one large network into two smaller networks for performance reasons. You can think of a bridge as a kind of smart repeater. Repeaters listen to signals coming down one network cable, amplify them, and send them down the other cable. They do this blindly, paying no attention to the content of the messages that they repeat.

In contrast, a bridge is a little smarter about the messages that come down the pike. For starters, most bridges have the capability to listen to the network and automatically figure out the address of each computer on both sides of the bridge. Then the bridge can inspect each message that comes from one side of the bridge and broadcast it on the other side of the bridge, but only if the message is intended for a computer that's on the other side.

This key feature enables bridges to partition a large network into two smaller, more efficient networks. Bridges work best in networks that are highly segregated. For example, suppose that the Sneetches networked all their computers and discovered that, although the Star-Bellied Sneetches' computers talked to each other frequently and the Plain-Bellied Sneetches' computers also talked to each other frequently, rarely did a Star-Bellied Sneetch computer talk to a Plain- Bellied Sneetch computer.

A bridge can partition the Sneetchnet into two networks: the Star-Bellied network and the Plain-Bellied network. The bridge automatically learns which computers are on the Star-Bellied network and which are on the Plain-Bellied network. The bridge forwards messages from the Star-Bellied side to the Plain-Bellied side (and vice versa) only when necessary. The overall performance of both networks improves, although the performance of any network operation that has to travel over the bridge slows down a bit.

Here are a few additional things to consider about bridges:

• Some bridges also have the capability to translate the messages from one format to another. For example, if the Star-Bellied Sneetches build their network with Ethernet and the Plain-Bellied Sneetches use Token Ring, a bridge can tie the two together.
• You can get a basic bridge to partition two Ethernet networks for about $500 from mail-order suppliers. More sophisticated bridges can cost as much as $5,000 or more.
• If you've never read Dr. Seuss's classic story of the Sneetches, you should.