setup default gateway with a route command

route command show and/or manipulate the IP routing table under Linux and UNIX oses.
Route manipulates the kernel’s IP routing tables. Its primary use is to set up static routes to specific hosts or networks via an interface after it has been configured with the ifconfig program. When the add or del options are used, route modifies the routing tables. Without these options, route displays the current contents of the routing tables.

Display default route
Following three-command display the current routing table:

1. # route
2. $ /sbin/route
3. $ /sbin/route -n

use -n option, to display numerical addresses instead of trying to determine symbolic host names (via dns or /etc/hosts file). This is useful if you are trying to determine why the route to your nameserver has vanished.

Add / setup a new route
The syntax is as follows:

route add default gw {IP-ADDRESS} {INTERFACE-NAME}

Where,
IP-ADDRESS: Specify router IP address
INTERFACE-NAME: Specify interface name such as eth0

For example if your router IP address is 192.168.1.254 type the following command as the root

# route add default gw 192.168.1.254 eth0

OR use hostname such as dsl-router:

# route add default gw dsl-router eth0