Different media access control methods may be required during the course of a single communication. Each network environment that packets encounter as they travel from a local host to a remote host can have different characteristics. For example, an Ethernet LAN consists of many hosts contending to access the network medium on an ad hoc basis. Serial links consist of a direct connection between only two devices over which data flows sequentially as bits in an orderly way.

Router interfaces encapsulate the packet into the appropriate frame, and a suitable media access control method is used to access each link. In any given exchange of network layer packets, there may be numerous data link layer and media transitions. At each hop along the path, a router:

The router in the figure has an Ethernet interface to connect to the LAN and a serial interface to connect to the WAN. As the router processes frames, it will use data link layer services to receive the frame from one medium, de-encapsulate it to the Layer 3 PDU, re-encapsulate the PDU into a new frame, and place the frame on the medium of the next link of the network.