Routers operate in the physical, data link and network layers of the OSI model. The Internet is a combination of networks connected by routers. When a datagram goes from a source to a destination, it will probably pass through many routers until it reaches the router attached to the destination network. Routers determine the path a packet should take. Routers relay packets among multiple interconnected networks. In particular, an IP router forwards IP datagrams among the networks to which it connects. A router uses the destination address on a datagram to choose a next-hop to which it forwards the datagram. A packet sent from a station on one network to a station on a neighbouring network goes first to a jointly held router, which switches it over the destination network. In fact, the easiest way to build the Internet is to connect two or more networks with a router. Routers provide connections to many different types of physical networks: Ethernet, token ring, point-to-point links, FDDI and so on.
• The routing module receives an IP packet from the processing module. If the packet is to be forwarded, it should be passed to the routing module. It finds the IP address of the next station along with the interface number from which the packet should be sent. It then sends the packet with information to the fragmentation module. The fragmentation module consults the MTU table to find the maximum transfer unit (MTU) for the specific interface number.
• The routing table is used by the routing module to determine the next-hop address of the packet. Every router keeps a routing table that has one entry for each destination network. The entry consists of the destination network IP address, the shortest distance to reach the destination in hop count, and the next router (next hop) to which the packet should be delivered to reach its final destination. The hop count is the number of networks a packet enters to reach its final destination. A router should have a routing table to consult when a packet is ready to be forwarded. The routing table should specify the optimum path for the packet. The table can be either static or dynamic. A static table is one that is not changed frequently, but a dynamic table is one that is updated automatically when there is a change somewhere in the Internet. Today, the Internet needs dynamic routing tables.
• A metric is a cost assigned for passing through a network. The total metric of a particular router is equal to the sum of the metrics of networks that comprise the route. A router chooses the route with the shortest (smallest value) metric. The metric assigned to each network depends on the type of protocol. The Routing Information Protocol (RIP) treats each network as one hop count. So if a packet passes through 10 networks to reach the destination, the total cost is 10 hop counts. The Open Shortest Path First protocol (OSPF) allows the administrator to assign a cost for passing through a network based on the type of service required. A route through a network can have different metrics (costs). OSPF allows each router to have several routing tables based on the required type of service.
