Simplified IP datagram routing algorithm

To route an IP datagram on the network, the algorithm shown in Figure 1 is used.

Figure 1. General IP routing algorithm

If the destination IP address equals one of my IP addresses, send the IP datagram on the local network. If not, send the IP datagram to the gateway that is corresponding to the destination IP address.

Using this general routing algorithm, it is easy to determine where an IP datagram is routed. Following is a simple example based on the configuration shown in Figure 1.

Machine A   IP Address = 192.9.200.1
 
Routing Table
 
Destination     Gateway
 
192.9.200.1     192.9.200.1   (Machine A's network interface)
 
9.0.0.0         192.9.200.2   (Route to the 9.n.n.n address is
                               via Machine D, 192.9.200.2)

Machine A sends a datagram to host 192.9.200.3 (Machine B), using the direct route, 192.9.200.1 (its own network interface). Machine A sends a datagram to host 9.67.32.2 (Machine C), using the indirect route, 192.9.200.2 (Machine D), and Machine D then forwards the datagram to Machine C.