Windows XP computers can be assigned IP addresses two ways: manually using static addresses or automatically using DHCP. If your computer is configured to use DHCP however, a problem can occur if the DHCP server is down when your computer needs to renew its lease. This happens typically if you reboot your computer when the DHCP is unavailable, and in this case Windows XP uses Automatic IP Address Allocation (APIPA) to automatically assign itself an address of the form 169.254.x.y. Once your computer has this address however, it typically can't communicate on the network anymore. To prevent this kind of situation from happening, you can assign your computer an alternate IP address to fall back on when your computer can't contact a DHCP server to lease an address. This is done by using the Alternate Configuration tab of the TCP/IP properties for your computer's Local Area Connection. A typical situation where you might want to use this might be if you have a laptop at home where you use DHCP to obtain an IP address from your ISP. If you take your laptop to work sometimes and your workplace is a small business that uses static addressing instead of DHCP, you can assign your computer a static address on the Alternate Configuration tab so that it can participate in your work network when present at work. Note that the Alternate Configuration tab is only visible when you've selected Obtain An IP Address Automatically on the General tab of your TCP/IP Properties.
Mitch Tulloch is President of MTIT Enterprises, an IT content development company based in Winnipeg, Canada. Prior to starting his own company in 1998, Mitch worked as a Microsoft Certified Trainer (MCT) for Productivity Point International. Mitch is a widely recognized expert on Windows administration, networking and security and has written 14 books and over a hundred articles on various topics. He has been repeatedly awarded Most Valuable Professional (MVP) status by Microsoft for his outstanding contributions in supporting users who deploy Microsoft platforms, products and solutions. Mitch is also a professor at Jones International University (JIU) where he teaches graduate-level courses in Information Security Management (ISM) for their Masters of Business Administration (MBA) program. For more information see http://www.mtit.com.