The very important task a bridge operates is frame filtering. If both the destination and source are on the same segment, it does not repeat the frame to the other code segment. A frame is forwarded to the other code segment, if it id destined to that part. Multicast and broadcast frames are also forwarded.
A bridge has a list for every segment that consists of physical addresses of the machine attached to that part. In this way a bridge finds on which code segment a destination device is attached.
As soon as a frame reachs to a bridge, it finds a source address from its header and automatically includes it in the list for that code segment. In this way a bridge creates up address lists. This is given in the figure below: