When two computers are directly connected, there is no need for routing because each computer knows exactly where to find the other computer. They communicate directly.

Networking computers allows many computers to communicate with each other. This requires each computer to have an IP address to identify its location to the other computers. This is much like a mailing address - you will not receive your postal mail at home if you do not have an address for people to send mail to. The routing table on a computer is much like an address book used to mail letters to people in that the routing table maintains a list of how to reach computers. Routing tables may also include information about the quality of service (QoS) of the route, and the interface associated with the route if the device has multiple interfaces.

Looking at routing as delivering letters is more simple than reality. In reality, routers loose power or have bad cabling, network equipment is moved without warning, and other such events happen that prevent static routes from reaching their destinations. When any changes such as these happen along a static route, traffic can no longer reach the destination — the route goes down. Dynamic routing can address these changes to ensure traffic still reaches its destination. The process of realizing there is a problem, backtracking and finding a route that is operational is called convergence. If there is fast convergence in a network, users won’t even know that re-routing is taking place.

The routing table for any device on the network has a limited size. For this reason, routes that aren’t used are replaced by new routes. This method ensures the routing table is always populated with the most current and most used routes—the routes that have the best chance of being reused. Another method used to maintain the routing table’s size is if a route in the table and a new route are to the same destination, one of the routes is selected as the best route to that destination and the other route is discarded.

Routing tables are also used in unicast reverse path forwarding (uRPF). In uRPF, the router not only looks up the destination information, but also the source information to ensure that it exists. If there is no source to be found, then that packet is dropped because the router assumes it to be an error or an attack on the network.

The routing table is used to store routes that are learned. The routing table for any device on the network has a limited size. For this reason, routes that aren’t used are replaced by new routes. This method ensures the routing table is always populated with the most current and most used routes — the routes that have the best chance of being reused. Another method used to maintain the routing table’s size is if a route in the table and a new route are to the same destination, one of the routes is selected as the best route to that destination and the other route is discarded.

Some actions you can perform on the routing table include: