 called an 'autonomous system'. A routing protocol that is used to move information within an autonomous system is called an Interior Gateway Protocol (IGP). A protocol that is used for exchange of information between different autonomous systems is called an Exterior Gateway Protocol (EGP). Figure 4.45 depicts the use of Interior and Exterior Gateway Routing Protocols. Various examples of the IGP are the RIP, OSPF and IS-IS (intermediate system-to-intermediate system) whereas the BGP is an EGP. These protocols work on the basis of a common philosophy of forwarding the packet to the next router, which is considered as the next hop. A route consists of a final destination network address and the IP address of the next hop to reach it. For example, for a destination network of 134.1.1.0, the next hop may be 10.132.1.1. Routing tables are defined in each of these routers. Routes are of two kinds, static and dynamic. Static routes are defined by the administrator. In dynamic routing, the protocol calculates routes. Dynamic routing algorithms adapt to changes in the network and automatically IP Routing Protocols select the best route. At each node, the destination address defined in the datagram is compared with the route defined in the router, the best next hop is decided and the packet is forwarded. Each node is responsible for forwarding the packet one hop at a time. The Internet protocol does not take care of error detection and/or correction. The ICMP (Internet Control Message Protocol), which is an integral part of any IP network, is responsible for this. Routing Information Protocol (RIP) The RIP is an Interior Routing Protocol developed by Xerox. The RIP works well in relatively small networks. Routing decisions are based on the hop count. A route is considered as the best route if it has the least number of hops to reach a final destination. However, it ignores other important factors like line utilisation and line speed. The RIP has serious limitations in large networks. It has a limit of allowing a maximum 