Computer Networking: Internet Protocols in Action

Chapter 4.3. Dynamic Routing with RIP

INTRODUCTION

In Exercise 4.2, we examined several end-to-end paths through the Internet using traceroute. In this exercise, we will examine how these paths are established and maintained.

On a path from source to destination, datagrams often pass through many intermediate router machines. Each router is responsible for directing datagrams one step closer to their final destination. Routers can be compared to a police officer directing traffic at an intersection. Cars enter the intersection from one direction and the police officer directs them to leave the intersection in the direction that takes them closer to their destination. Routers have multiple network interfaces that act like the street leaving an intersection. Datagrams arrive on one of the router's network interfaces and are retransmitted from another network interface, ideally the one closest to its destination.

You may wonder how a router knows which way a datagram should go. In some cases, a human operator manually configures the router with a set of static rules. For example, the router shown in Figure 4.3.1 would need only two simple rules. If a datagram is destined for an IP address in the 145.45.32.0/24 network, then it should be sent out the interface labeled B; otherwise it should be sent out the interface labeled A. (Note: 145.45.32.0/24 means that the last 8 bits are free to be allocated to hosts within the network and that the remaining 24 bits indicate the network ID. The ...