Choosing a gateway address on Linux machines is an easy task to perform. Let’s consider a network consisting of three Linux machines as Figure 1 shows.
Let’s consider following subnet and IP addresses:
- Subnet 1: 10.0.0.0/24, IP Address 1: 10.0.0.1, and IP address 2: 10.0.0.2.
- Subnet 2: 11.0.0.0/24 and IP Address 3: 11.0.0.1.
- Subnet 3: 12.0.0.0/24 , IP Address 4: 12.0.0.1, and IP address 6: 12.0.0.2.
- Subnet 4: 13.0.0.0/24 and IP Address 4: 13.0.0.1.
Run “route -n” command to check the current routes on each PC. Then use “
Since PC 1 is only connected to subnet 1, we first add a route to subnet 1 using following command:
$ route add -net 10.0.0.0 netmask 255.255.255.0 gw 10.0.0.1
To connect PC 1 to other subnets on the network, we either need to add other subnets addresses to the routing table one by one or add a default gateway to send all the packets not intended for Subnet 1 to that gateway.
$ route add default gw 10.0.0.2
Similar to PC 1, we can add the routes to other PCs on the network based on the tables in Figure 1.
On PC 2:
$ route add -net 10.0.0.0 netmask 255.255.255.0 gw 10.0.0.2
$ route add -net 11.0.0.0 netmask 255.255.255.0 gw 11.0.0.1
$ route add -net 12.0.0.0 netmask 255.255.255.0 gw 12.0.0.1
$ route add -net 13.0.0.0 netmask 255.255.255.0 gw 13.0.0.1
And the default gateway is for all the packets to other networks which are not directly connected to PC 2:
$ route add default gw 12.0.0.2
PC 3 is directly connected to Subnet 3, but it is not directly connected to Subnets 1, 2, and 4. To add the routes, we use the following commands:
$ route add -net 12.0.0.0 netmask 255.255.255.0 gw 12.0.0.2
$ route add -net 10.0.0.0 netmask 255.255.255.0 gw 12.0.0.1
$ route add -net 11.0.0.0 netmask 255.255.255.0 gw 12.0.0.1
$ route add -net 13.0.0.0 netmask 255.255.255.0 gw 12.0.0.1