Router Packet Forwarding Decision

When a host going to sends a packet to another host; it will use host routing table where to send the packet. If the destination host is not on the same network, as a result the packet will be forwarded to the default gateway. When a packet arrives at the default gateway, which is generally a router. So the routers consult its routing table to decide where to forward this packet. The routing table of a router contains information about:

Directly connected routes

These routes come from the router active interfaces. Routers add a directly connected route when an interface is configured with an IP address and is activated. Each of the router's interfaces is connected to a different network segment.

Remote routes

These routes come from remote networks connected to other routers. Routes to these networks can be manually or dynamically configured on the local router by the network administrator.

Default route 

Routers also use a default route as a last option if there is no other route to the desired network in the routing table.

The figure identifies the directly connected networks and remote networks of router-1. Networks in the red rectangles are directly connected networks for router-1 and network in the blue rectangles are a remote network for router-1 and vice versa.