Bridges are mainly used to connect the multiple LANs which operate in the Data Link layer to construct a larger LAN. It is responsible to filter traffic passing through it and provides the extra security to each LAN. The basic function of the Bridge is to transfer the frames from one LAN (LAN A) to another LAN (LAN B) as shown below in the figure. Some time bridge work as repeater but it is more intelligent then repeaters. It examines which computer is connected to which LAN and it learn whether to transmit each frame.

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Bridge examines the header of each incoming frame and based on this information bridge decides whether to discard or forward this frame. Bridge stores the MAC address of received frame, keep record on which port it has received, and which address belong to which computer on the LAN. This process is known as Learning.