8.4 The Network Layer
8.4.4 The concept of default gateways and
subnetworks
The connection of a LAN to a WAN is achieved through a router; therefore, LANs can be segmented by routers. The interface of a router, which resides on a LAN, is called the default gateway. The default gateway is the location where all non-local network traffic that has no specific route to a destination is sent. The default gateway acts as an entry and exit point of a
subnetwork. 

When sending data to a remote subnetwork, the host sends the packet to the initial router specified as its default gateway. The router receives the packet,
then must determine whether the destination location is on one of its local networks or send the data to another router for delivery.