A LAN bridge connects two or more LANs at layer two in the OSI network model. The LAN bridge receives packets from a LAN segment connected to one port and forwards them to another LAN segment connected to a different port. While a LAN bridge serves the purpose of extending network range, it also relieves the problem of congestion that multiple devices can cause on a single Ethernet segment.

The LAN Bridge — Bridging the Gaps in Network Communications

LAN bridges employ varying mechanisms to deliver their functionality. A simple LAN bridge regulates the transmission of frames to avoid congestion on the network. A learning LAN bridge remembers (learns) the Ethernet address of each frame it receives, in order to record  which devices are connected to each port. The learning bridge can then examine the destination address of each received frame to determine whether or not it should be forwarded to another part of the network. This selective forwarding improves the efficiency of communications across the network.

While bridges provide services similar to those offered by routers and repeaters, there are some significant differences. Routers, like LAN bridges, act as agents to receive and forward messages. Unlike a router, however, a LAN bridge has no network-layer address. The LAN bridge is transparent to both client and server workstations. Repeaters, are like LAN bridges in that they also transmit information across an Ethernet network. But having no memory, a repeater will retransmit all the data it receives, including any frames that cause collisions. Unlike a repeater, A LAN bridge has the memory and intelligence to alleviate collisions when forwarding Ethernet frames.

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