IP Routing

IP routing is the process use by a router to forward packets towards destination network as found in the routing table, which is created by routing algorithms or routing protocols.
A router makes decisions based upon the destination IP address of a packet. Routers don’t care about hosts in a network, but networks and the best possible way to reach them. All network devices along the way use the destination IP address to send the packet in the right direction to reach its destination. To make the correct decisions, routers must learn how to reach remote networks. Routers use either dynamic routing - this information is learned from other routers, or static routing - a network administrator configures information about remote networks manually.
Although it’s deemed necessary for every device in the network to be assigned a unique IP address, but this is not sufficient for them to communicate. It’s the work of a router to acts as a sort of traffic director to all packet destined to a destination.
In summary, routers route traffic to all networks in your internetwork. For a router to be able to do this, it must know the following:

i.    Destination address

ii.   Possible routes to all networks

 iii. Neighboring routers from which it will learn about remote networks

iv. The best route to reach a network

v. How to maintain and verify routing information.

 

Example below shows how a router uses information in its routine table to make decisions:

 

 

 

R2#show IP route
[Output omitted]
Gateway of last resort is not set
C   192.168.1.32/27 is directly connected, fastEthernet0/1
C   192.168.1.0/27 is directly connected, fastEthernet0/2
C   10.10.1.0/30 is directly connected, serial 0/0/0
The C in the routing table means the networks are directly connected. The remote networks are not found and displayed in the routine table because, we have not added a routing protocol – such as RIP, EIGRP, OSPF etc. etc or configured Static routes.
Looking at the output above, when the network router receive a packet with the destination address of 192.168.1.10, the router will send the packet to interface fastEthernet0/2, and this interface will frame the packet and then send it out on the network segment to Network B.