Bridges receive a packet and then transmit it on the other side. Introduce a delay of an additional frame transmission time. Collisions are not seen on the other side. Bridges learn the location of hosts. Routers and Gateways Network level Store and forward frames between dissimilar networks.
Routers are visible to the network software.
Frames must be addressed to a gateway. May perform protocol conversion. Cycles of bridges have to be avoided. A packet could circulate forever. Sometimes it is advantageous to have extra bridges for robustness and performance.