A bridge is a Data Link layer device that connects two or more LAN segments and forwards frames between them selectively, based on the destination MAC address — rather than blindly repeating everything (like a hub/repeater) or routing based on IP (like a router).

Key roles:

1. Filtering — examines each frame's destination MAC address and forwards it only to the segment where that address actually resides, keeping traffic local to its own segment when the destination is nearby.
2. Learning — builds a forwarding/MAC address table automatically by observing the source addresses of incoming frames, associating each address with the port it arrived on.
3. Segmenting collision domains — by splitting one large shared LAN into smaller segments, it reduces collisions and increases effective usable bandwidth.
4. Extending the LAN — links physically separate LAN segments (possibly using different media) into a single logical network, transparently to the higher layers.