Direct Routing

When a machine (host or gateway) wishes to send a datagram to another
machine in the same physical network, the process is simple. The datagram is
encapsulated in a frame, its physical address is determined from its IP address
(either directly or using the ARP protocol), and it is sent using the network
hardware.
The obvious question is how to know if the destination machine is
within the same physical network. The answer is trivial. As has been seen,
every IP address is made up of a prefix specifying the network and a suffix
identifying each specific host. If the network prefixes identified by the origin
and destination machines coincide, then they are in the same network.