|  | 11.5 | The Context of Different Routing Protocols
| 11.5.4 | LAN-to-WAN routing
The network layer must relate to, and interface with, various lower layers for LAN-to-WAN traffic. As an internetwork grows, the path taken by a packet may encounter several relay points and a variety of data link types beyond the LANs. For example, in the Figure, the following takes place: A packet from the top workstation at address 1.3 must traverse three data links to reach the file server at address 2.4, shown on the bottom. The workstation sends a packet to the file server by first encapsulating it in a token-ring frame addressed to Router A. When Router A receives the frame, it removes the packet from the token-ring frame, encapsulates it in a Frame Relay frame, and forwards the frame to Router B. Router B removes the packet from the Frame Relay frame and forwards it to the file server in a newly created Ethernet frame. When the file server at 2.4 receives the Ethernet frame, it extracts and passes the packet to the appropriate upper-layer process. Routers enable LAN-to-WAN packet flow by keeping the end-to-end source and destination addresses constant while encapsulating the packet in data link frames, as appropriate, for the next hop along the path.