Ever notice that computers seem to talk faster with each other when communicating across a local-area network (LAN) versus talking with a computer in another city, state, or country? The reason is cost: It costs a lot of money to transfer traffic across the country as opposed to transferring that same rate of traffic to a computer in the next room.

LANs are speedy and relatively low-cost. Want to transmit 100 Mbps of traffic between two computers in the same room? Connect an Ethernet cable between two computers with fast Ethernet cards at minimal cost.

However, it is impractical to do the same between two computers in two different states. Instead, companies known as service providers spend gobs of money building network infrastructures called wide-area networks (WANs). By combining traffic from multiple customers across their networks, they can sell network access. But this access can be expensive, and bandwidth is at a premium.

Moving traffic across the street has requirements far different than moving traffic across the world. In Part VI, you learn why different means deliver traffic for networks that cover different areas.