Routing Table

In this tutorial, we are going to see What is the Routing Table and how it works? The routing table is a table of correspondence between the address of the targeted machine and the next node to which the router must deliver the message. In fact, it is enough that the message is delivered on the network that contains the machine, so it is not necessary to store the complete IP address of the machine: only the network identifier of the IP address needs to be stored.

The routing table is therefore a table containing pairs of addresses. Here is a simplified view of what a routing table might look like:
 

 
 
Thus thanks to this table, the router, knowing the address of the destination encapsulated in the message, will be able to know on which interface to send the message (that is to say which network card to use), and to which router, which is directly accessible on the network to which this card is connected, to deliver the datagram.

This mechanism of knowing only the address of the next route leading to the destination is called next-hop routing.

However, it is possible that the destination belongs to a network not referenced in the routing table. In this case, the router uses a default router (also called default gateway).

The message is thus delivered from router to router by successive hops until the destination belongs to a network directly connected to a router. The router then delivers the message directly to the target machine…

In the situation of static routing, it is the administrator who updates the routing table.

In the situation of dynamic routing, on the other hand, a protocol called routing protocol allows the table to be updated automatically so that it contains the optimal route whenever it is required.
 
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