Here's a question are teacher presented us the other day: Why do Wide Area Networks ONLY focus on the physical and data link layers? I cant just outright ask the teacher for the answer because its for bonus points. So I figured i'd come to you guys. I understand WANs use the physical layer by sending electrical pulses over telephone lines and what not. And I understand WANs use the data link layer by using mac addresses of devices such as the CSU/DSU. But, why isn't the network layer included? Routers, a key part of WANs, work off of this layer to route information. Also, packet-switched networks within the telephone company are based on the PDU of the network layer, the PACKET! What am I missing here, or is this just a trick question that my teacher has used just to get the gears in my head turning? Please help!