If a NIC wants to communicate with another NIC that is not in the same network, then routing is required. In each IP network that has the ability to connect out of the network, a particular address is called the “default gateway”. All traffic that does not have a destination local to a network is encapsulated and directed to this “default gateway”. The default gateway, in return, attempts to deliver the traffic over internet connections to the destination network.
Typically, the default gateway is a NIC of a router.