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A bridge is a type of packet switch which is fundamentally different from a router. It effectively unifies two otherwise separate physical networks; a host sending a packet cannot tell whether the destination network interface is on the same physical network as it is, or on another one, which is connected to its physical network through a bridge. The two physical networks must thus share a unified address space.

The bridge thus operates on the protocol stack layer beneath the internetworking layer; bridges usually have no knowledge of that layer. A bridge operates mostly invisibly, and do not have its own address on the physical networks (although it may have one for operation and maintenance purposes).

A bridge thus needs no configuration; one merely plugs it in, and as soon as it is powered on, it starts to operate. (Hence the saying that "The advantage of a bridge is that it looks like a wire, but it isn't. The disadvantage of a bridge is that it looks like a wire, but it isn't.")