|  | 3.1 | Basic LAN Devices
| 3.1.7 | Bridges

A bridge is a Layer 2 device designed to connect two LAN segments. The purpose of a bridge is to filter traffic on a LAN, to keep local traffic local, yet allow connectivity to other parts (segments) of the LAN for traffic that has been directed there. You may wonder, then, how the bridge knows which traffic is local and which is not. The answer is the same one that the postal service uses when asked how it knows which mail is local. It looks at the local address. Every networking device has a unique MAC address on the NIC, the bridge keeps track of which MAC addresses are on each side of the bridge and makes its decisions based on this MAC address list.
The appearance of bridges varies greatly depending on the type. Although routers and switches have taken over much of the bridge's functions, they nonetheless remain important in many networks. To understand switching and routing you must first understand bridging.
The bridge symbol, which resembles a suspension bridge, is shown in the graphic. Traditionally, the term bridge refers to a device which has just two ports. However, you will also see references to bridges with 3 or more ports. What really defines a bridge is its layer 2 filtering of frames and how this is actually accomplished. Just as was seen in the case of the repeater/hub combination, there is another device that is used for multiple bridge connections. This device is covered on the next page.