A logical segment of an ethernet network. The bridge must contain at least two interfaces, but may sometimes have many more. Each interface on the bridge is put into promiscuous mode, where it watches everything that happens on each physical side of the network. If a packet on one side of the network is destined for an interface on the other, the bridge broadcasts the packet onto the opposite physical network. This trick is accomplished through a table of learned (or discovered) ethernet MAC addresses which designate which interfaces are on which sides of the complete logical network.

Ethernet bridges are useful for attaching two networks of a different architecture, such as 10baseT and 100baseT, or 1000baseT, (!). It could be taken a step further to integrate a coax or BNC network onto a twisted pair network, where a fancy hub is not available. Many sysadmins like ethernet bridges, as it resets the hub stacking rule, and generally reduces latency across the complete logical network. Not to mention, if you have a group of computers which are quite noisy amongst themselves (i.e. a bunch of Appletalk or IPX machines), it is nice to seperate them, and cut the network load on the complete virtual network.

Cool, huh?