Network bridges

A network bridge connects multiple network segments at the data link
layer (layer 2) of the OSI model. Bridges do not promiscuously copy
traffic to all ports, as hubs do, but learns which MAC addresses are
reachable through specific ports. Once the bridge associates a port
and an address, it will send traffic for that address only to that
port. Bridges do send broadcasts to all ports except the one on which
the broadcast was received.

Bridges learn the association of ports and addresses by examining the
source address of frames that it sees on various ports. Once a frame
arrives through a port, its source address is stored and the bridge
assumes that MAC address is associated with that port. The first time
that a previously unknown destination address is seen, the bridge will
forward the frame to all ports other than the one on which the frame
arrived.

Bridges come in three basic types:

1. Local bridges: Directly connect local area networks (LANs)
2. Remote bridges: Can be used to create a wide area network (WAN)
link between LANs. Remote bridges, where the connecting link is slower
than the end networks, largely have been replaced by routers.
3. Wireless bridges: Can be used to join LANs or connect remote
stations to LANs.

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