Linux Networking Cookbook

6.3. Setting Up a Simple Local Router

Problem

You have a single shared Internet connection, and your LAN is divided into a number of subnets. You want your subnets to be able to communicate with each other. What do you have to do to make this magic occur?

Solution

Not much. All it takes is a single router, and all of your subnets connected to it. Suppose you have these three subnets:

• 10.25.0.0/16

• 172.32.0.0/16

• 192.168.254.0/24

You router needs to have three network interfaces with one address on each network segment:

• eth0 = 10.25.0.10

• eth1 = 172.32.12.100

• eth2 = 192.168.254.31

Each subnet has its own switch, which is connected to your router, like Figure 6-2.

Figure 6-2. Local subnets connected to a single router

Then, turn on IP forwarding on your router. You can do this from the command line:

	# echo 1 > /proc/sys/net/ipv4/ip_forward

This does not survive a reboot, so you can set it permanently in /etc/sysctl.conf, and then start it immediately:

	##/etc/sysctl.conf
	net.ipv4.ip_forward = 1

	# sysctl -p

Next, assign these three addresses as the default gateways for the hosts on each network. All computers in the 10.25.0.0/16 will use 10.25.0.10 as their default gateway, and I think you can extrapolate what the other two networks will use for their default gateways.

Once this is done, your three networks will be able to pass TCP/IP traffic back and forth with ease.

Discussion

You don't have to use addressing ...