Addressing DHCP Server Inaccessibility from Client Systems

Every now and then I get email from readers who are having DHCP-related problems on their networks. Because this column targets client-side Windows OSs, I don't usually address DHCP problems here (although I do email a response to those readers). However, one relatively common DHCP problem—that of the DHCP server being down or inaccessible—is easily addressed on the client side in Windows XP. Many Windows Client UPDATE readers apparently use as few DHCP servers as possible to service large network enterprises; sometimes client systems can't reach a DHCP server, which means the client system loses access to network resources because the DHCP server hasn't been able to assign an IP address. My readers' messages tell me that the most common scenario is when 802.11b wireless clients can't reach a DHCP server on the wired side of the network.

Many network infrastructure problems can be responsible for disappearing DHCP servers. However, XP offers a client-side feature that lets you keep your clients running even if they can't find a DHCP server. If you've booted up an XP or Windows 2000 computer that wasn't attached to a network but was configured for DHCP, you undoubtedly noticed that the OS assigned the computer an IP address in the 169.254.xxx.xxx address block. This address is known as an automatic private address (officially, it's an Automatic Private IP Addressing—APIPA—address); the address assignment is the default XP and Win2K behavior when the client system can't find a DHCP server. In XP, you can use the TCP/IP Alternate Configuration tab in the TCP/IP Properties dialog box to change this default behavior by configuring an alternate IP address. Because the automatic private address doesn't let the OS configure gateway or DNS information, using the Alternate Configuration tab to configure an alternate address is the only way you can automate an IP address assignment that allows continued network access beyond the local subnet in the event of a DHCP server failure.

Many people configure an alternate IP address for wireless networking and mobile computing; doing so provides a simple way to let users move between different wireless networks without having to reconfigure the IP address every time they switch networks. For wireless use, it's important to note that configuring an alternate IP address doesn't affect the security model that each network uses; the primary security model setup for the client system's wireless card will be in effect. Because of this limitation, you might want to maintain multiple wireless PC Cards if you switch between highly secure wireless networks. Each network's security configuration is specific to the network adapter. Because each adapter is unique (i.e., is identified to the OS by its media access control—MAC—address), you can configure adapters of the same brand specifically for each of your secure wireless networks.

To access the Alternate Configuration tab, go to My Computer, open View Network Connections, right-click the connection you want to modify, and click Properties on the context menu. Select Internet Protocol on the General tab and click Properties, then click the Alternate Configuration tab. If you typically use DHCP, you can exclude an IP address range from the DHCP server's control and use that address range to provide alternate IP addresses. Doing so will help prevent duplicate IP address error messages.