It is common to have interconnected networks. For example, different departments at work or school may be on separate networks. When a device on one network wants to communicate with a device on the other network, it needs to send packets through a device which is common to both networks. This device is called a gateway and its function is to route packets to and from different networks.

The operating system maintains a table called the routing table, which contains the information on how packets are to be forwarded through machines on the network. The route command displays the routing table:

$ route 
Kernel IP routing table 
Destination      Gateway   GenmaskFlags  Metric  Ref  UseIface 
192.168.0.0         *      255.255.252.0  U     2      0     0wlan0 
link-local          *      255.255.0.0    U     1000   0     0wlan0 
default          p4.local  0.0.0.0        UG    0      0     0wlan0 

Alternatively, you can also use this:

$ route -n 
Kernel IP routing table 
Destination   Gateway      Genmask       Flags Metric Ref  UseIface 
192.168.0.0   0.0.0.0      255.255.252.0   U     2     0     0   wlan0 
169.254.0.0   0.0.0.0      255.255.0.0     U     1000  0     0   wlan0 
0.0.0.0       192.168.0.4  0.0.0.0         UG    0     0     0   wlan0 

Using -n specifies to display the numeric addresses. By default, route will map the numeric address to a name.

When your system does not know the route to a destination, it sends the packet to a default gateway. The default gateway may be the link to the Internet or an inter-departmental router.

The route add command can add a default gateway:

# route add default gw IP_ADDRESS INTERFACE_NAME 

Consider this example:

# route add default gw 192.168.0.1 wlan0