Understanding Routing Table

Routing Table

We all know, that the router’s primary function is to forward a packet towards its destination address. A router have physical interfaces which connects to other network devices. 

The routing table contains a list of specific routing destinations or pair of IP addresses in its memory. Sometimes it is also called routing information base of RIB. When the router receives a packet of data, it references the routing table to know where to send that data. The device kernel reads the routing table. Each entry of row in the routing table defines a route. This route is in two types such as network routes and host routes. So the router does not have to send any query or to wait for a routing input from user to send packets each time.

Network routes says, how to reach to a specific network ID within the internetwork. Whereas the host route provides information to reach a particular host on a network.

All IP enabled devices use routing table. Means the routing table keeps the information about the entire topology of the network such as IP address, gateway address, port number, subnet mask, interface, metric.

Metrics: if in doubt about metrics, it is a number describes the best route or it helps in router to choose best route among multiple routes to the destination. It helps dynamic routing protocols like, RIP, OSPF, EIGRP to decide which is shortest and best path to reach the destination. 

Static routing uses a routing table which can preconfigured manually, where all entries will remain the same unless they are changed manually. It can be best used for small networks, hosts. For many small office work space, people use window system. If you want to add a static route to the table, the below syntax can be used:

Route ADD destination_network MASK subnet mask gateway_ip metric_cost

In case of Dynamic routing, protocols allow routers to get information from other (peer) routers on the network such as the routers can be configured to learn IP destinations from other routers in order to update routing table entries without user intervention. For more information in dynamic routing please refer to EIGRP, OSPF, BGP and RIP post. Dynamic routing is suitable for larger internetworks.

A sample routing table provided as below:

Network Destination	Subnet Mask	Gateway	Interface	Metric
101.X.X.0	255.255.255.0	10.X.X.2	Eth01	1
Default	0.0.0.0	10.X.X.3	Eth02	0
127.X.X.9	255.255.255.0	10.X.X.4	S0	291
10.X.X.8/16	255.255.255.0	10.X.X.1	GE1/0/0	11

-DR