The final thing that we are going to look at doing with the ifconfig command is what is known as plumbing a device.

Plumbing a device or plumbing a network means putting two IP addresses on one link.

In a sense what this looks like is, that on an interface you can have 192.168.0.1 on eth0 and on eth0:0 you can also have 172.16.5.1.

This would allow us to have a single Linux machine to act as a gateway between two different networks. If we draw this out logically we will have networks 192.168.0 and 172.16.5, and in the middle we will have our Linux machine. This will allow us to connect two completely separate networks and force our Linux machine to be the router.

If we take this to our logical next step, when you set up your Linux machine at home and you connect your modem to it you are essentially doing exactly the same thing; turning your Linux machine into a router. The internal eth0, (your Ethernet card) might have an IP address of 10.0.0 5 and your ISP, Internet Service Provider, might give you a separate IP address of 196.6.25.15, and the only way you can communicate between other hosts internally on your LAN and the Internet, is by forcing your Linux box to be a router.

How many times can we plumb an interface?

Well certainly more than you are going to need to, but I think the maximum limit is 255.

Linux can quite easily be used as a router, equally it can be used as a packet shaper or as a means of creating virtual LAN's or VLAN's, but this is beyond the scope of this course.

To plumb your device, you may run the command:

ifconfig eth0 192.168.0.1 netmask 255.255.255.0
ifconfig eth0:1 172.16.5.1 netmask 255.255.255.0
            

These commands would plumb the interface. Notice that the eth0:1 gives the interface the second IP address. Using any number in place of the '1' would also work.