Have you ever been stuck with having to connect to two or more different networks having different IP addressing schemes? At the locations which I support, I have to do this regularly as the Hospital Information System runs on a different IP addressing scheme and the Internet on the other -through the same switch. This means that I can’t use the resources on the Internet for troubleshooting assistance when I’m working on the HIS system. This was setup this way about 7 years ago when concerned people felt that connecting a system to the Internet gets your system infected. This never brought down the infection rate as much of the software was unlicensed and no security patches, updates or service packs were ever applied unless an application refused to install and demanded a certain patch level. The infections of course came first from viruses which had been immortalized on writable CD’s and later on from other media such as DVD’s, USB hard drives and pen drives. Needless to say, without the Internet, the anti-virus software itself lacked the latest virus definition files for reliable detection of viruses. This did however keep users from idling away their time away on the Internet. The most obvious workaround would be to keep changing your IP addresses as and when you require to connect to the other network. This is however impractical when you need to do so every 2-3 minutes as part of troubleshooting. Even if you need to do so only once a day it is still an inefficient way of computing as it is best to automate as much of your work as possible .In addition to this, I must also mention that as of now on Ubuntu 10 (Lucid Lynx) and probably even on the previous version, changing your network settings from the GUI mostly does not work. Even restarting the networking from a shell prompt has no effect and I’ve had to resort to a neat Windows trick -restart your computer 
Windows, Linux and probably the Mac too have provision for adding multiple IP addresses to a network adapter. You can add multiple IP addresses to servers as well. In this post we will be covering the Windows and Ubuntu Linux Desktop operating systems only.
In Windows Vista & 7, Go to network & sharing center > Change Adapter settings or View Network Connections and double click the required network adapter. Double Click Internet Protocol Version 4 (TCP/IPv4) and click on the Advanced tab. You can Add a new IP Address and subnet mask here. You can add two or more IP addresses here out of which one can be obtained via DHCP. In Windows XP, double click your network icon and go to properties and do the same thing or right click your adapter icon and click properties.
Advanced Network Settings on Windows XP, Vista & 7
On Ubuntu Linux, right click the network connections icon on the top right hand side and select Edit Connections. Go to the IPV4 Settings tab of the required network adapter and add a new IP address there. I prefer to leave the gateway blank as we have only one gateway. As mentioned above, if the settings don’t work, you will need to restart Ubuntu 
Once again, you can choose to obtain one of the IP addresses via DHCP
Adding an additional IP address in Ubuntu
Here is a post (external link) on setting up a linux server to have multiple IP addresses on a single network card.
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