t enough to reroute the traffic automatically. Routers make decisions about how to route the data or packets, they decide which path is best, and then they use that best path. Routers work at the network layer, Layer 3, of the OSI model, which allows them to switch and route packets across multiple networks. Routers build comprehensive sets of best routes to all locations that they know about. These routing tables can be quite large for complex networks. Routers can share status and routing information with one another and use that information to bypass slow or malfunctioning connections. Routing is the main process that an Internet host uses to deliver packets. As shown in Figure 8.4, the Internet uses a hop-by-hop routing model, which means that each host or router that handles a packet examines the destination address in the packet's IP header, computes the next hop that will bring the packet one step closer to its destination, and delivers that packet to the next hop, where the pro- cess is repeated. To make this happen, routing tables must match destination addresses with next hops, and routing protocols must determine the content of these tables. The next hop is determined from looking up the next hop in the rout- ing table, and routers must build and maintain these tables containing the "best routes." This routing is most often dynamic, scalable, and robust (assuming that the network is designed correctly). Forwarding Table for Router A The More 152.1.1.1/32, NH = C Specific Route 152.1/16, NH = B Packet Packet Packet 152.1/16 152.1.1.1 Destination Address = 152.1.1.1 Figure 8.4 Basic IP routing Chapter 8 The Internet and IP Infrastructures The Internet and the public switched telephone network (PSTN) operate quite differently from one another: The Internet uses packet switching, where there's no dedicated connection and the data is fragmented into packets. Packets can be delivered via differ- ent routes over the Internet and reassembled at the ultimate destination. Historically, back-office functions such