Because they operate at the network layer, they are fully aware of different network technologies, and can overcome such problems as interconnecting different network technologies. Also known as level 3 routers. The idea behind bridges is to transparently interconnect LANs. By transparently, we mean that a host treats all other hosts as if they were attached to the local LAN, when in fact, they may be connected to remote LANs. Bridges operate in promiscuous mode , reading every frame on a LAN, regardless of who it is addressed to.
Bridges run into immediate difficulties.
It is simply not possible for an ethernet to carry large frames originating from token ring LANs. Bridges will have to buffer frames. When congested, bridges must discard frames. No, because the bridge may be up, although the actual destination host may not be. In addition, host software that assumes hosts are on the directly connected LAN may fail.