Routing Logic

Routers are used to Communicate Systems from different TCP/IP areas.

Imagine that you are in Head Office, if you want to communicate with a person who is inside your Network (LAN) then Routers are not necessary unless your network is Sub-netted. But if you are trying to communicate with your Branch Office which is located 500 Km out from the Head Office, now you need the help of Routers.

Whenever any packet comes to the router where the destination IP address is not its own IP address the routers overwork of seeing the routing table and then forward the packet to the concern person. So the routers do IP routing.

After we configure the router, we configure all the systems in our network with a default gateway which is the router’s IP address. When the default gateway is added a default route gets added in the routing table. When the IP protocol does route it applies a default route only when there is no specific route available for that network destination. Routers also route the packet only when they have a matching route in the routing table, if the route is not available then routers drop the packet and generates an ICMP error message.

Before the transmission takes place data's were divided into several packets, each packet may take a different route to reach the destination, which we called as packet switching.

Click the image to view How the packet travels.