The Complete Handbook of Network Bridges

Chapter 1: Understanding Network Bridges

A network bridge is a device that connects two or more network segments and passes traffic between them based on the hardware addresses, called MAC addresses, of the devices on each side. This simple definition, however, hides the practical utility that has kept the bridge a relevant piece of equipment for decades. In a large and trafficked network, a bridge is not just a wire with a fancy jacket. It is a decision point. It looks at each chunk of data, called a frame, and decides whether that frame needs to cross to the other side. If the destination is on the same side as the sender, the bridge drops the frame. If the destination is on the other side, the bridge forwards it. This process of filtering and forwarding is what makes a bridge fundamentally different from a hub, which blindly repeats all data out every port.

The best way to ground this is with a concrete scenario. Consider a company with two buildings, Building A and Building B, about 200 meters apart. Building A houses the main office servers and internet connection, while Building B is a workshop with a dozen computers and a networked inventory printer. A single long Ethernet cable could be run from a switch in Building A to a switch in Building B, and it would work. But the traffic from the workshop printers, the office backups, and the inventory system would all compete on that single link, and