Examining the Routing Process

By Edward Tetz from Cisco Networking All-in-One For Dummies

The routing process starts with a source computer that makes the first decision in the routing steps, which is illustrated below. Not shown in the figure is the implicit route that each router would have for any network segments on which the routers interfaces are directly connected. The routing process goes like this:

The source computer at 192.168.1.25 wants to send data to the computer at 192.168.100.75.

The source computer consults its local routing table, which, for simplicity, has only one routing entry for its default gateway (Router1).
As the data arrives at Router1, Router1 looks at the destination address of the IP packet.

It then scans through its own routing table to determine that the data should be sent off the network over Router2 as a route to the destination.
As the data arrives at Router2, Router2 looks at the destination address of the IP packet.

Router2 determines that it does not have a specific routing entry to the destination network, but does have a default route or gateway of last resort, so Router2 passes the data on to Router3.
The data arrives at Router3.

Router3 has two entries in its routing table: one for the network of 192.168.100.0 through Router4 and one for the host (denoted by /32 in the routing table) of 192.168.100.75 through Router5. In this case, Router3 chooses the most specific routing table match to the destination address. Router3 then sends the data to Router5.
Router5 looks at the destination address of the IP packet.

Router5 reviews its routing table, and finds that it is directly connected to the destination network. In that case, it sends the data directly through the interface configured for the network 192.168.100.0, which gets the data to 192.168.100.75, the final destination.