1-11 IP Routing OverviewGo to these sections for information you are interested in:z IP Routing and Routing Tablez Routing Protocol Overviewz Displaying and Maintaining a Routing TableThe term “router” in this document refers to a router in a generic sense or a Layer 3 switch.IP Routing and Routing TableRoutingRouting in the Internet is achieved through routers. Upon receiving a packet, a router finds an optimalroute based on the destination address and forwards the packet to the next router in the path until thepacket reaches the last router, which forwards the packet to the intended destination host.Routing TableRouting tableRouting tables play a key role in routing. Each router maintains a routing table, and each entry in thetable specifies which physical interface a packet destined for a certain destination should go out toreach the next hop (the next router) or the directly connected destination.Routes in a routing table can be divided into three categories by origin:z Direct routes: Routes discovered by data link protocols, also known as interface routes.z Static routes: Routes that are manually configured.z Dynamic routes: Routes that are discovered dynamically by routing protocols.Contents of a routing tableA routing table includes the following key items:z Destination address: Destination IP address or destination network.z Network mask: Specifies, in company with the destination address, the address of the destinationnetwork. A logical AND operation between the destination address and the network mask yieldsthe address of the destination network. For example, if the destination address is 129.102.8.10and the mask 255.255.0.0, the address of the destination network is 129.102.0.0. A network maskis made of a certain number of consecutive 1s. It can be expressed in dotted decimal format or bythe number of the 1s.z Outbound interface: Specifies the interface through which the IP packets are to be forwarded.
