OSI Layer 1,2,3,4

Physical layer: transporting bits between two machines
Data link layer: MAC addresses and switching
Network layer:Packets, ip addresses and routing
Transport Layer: UDP segments and windowing

Bridges

A Bridge is a device that connects two local-area networks (LANs), or two segments of the same LAN. Unlike a router, bridges are protocol independent. They simply forward packets without analyzing and re-routing messages. Bridging occurs at layer 2 of the OSI model which is the data link layer. This means a bridge cannot read IP addresses, but only the outermost hardware address of a packet.