Windows XP Professional Edition and Windows 2000 Professional have a networking feature that automatically assigns a private IP address from the Internet Assigned Numbers Authority (IANA)—reserved Class B network. This feature is called the Automatic Private IP Addressing (APIPA) functionality. APIPA automatically starts if an XP Pro or Win2K Pro system is configured as a DHCP client and for some reason the DHCP lease can't be assigned or renewed. This problem could occur because of unexpected DHCP server behavior, such as the DHCP server being down, unresponsive, or unreachable. In these scenarios, an XP Pro or Win2K Pro system configured as a DHCP client will autoconfigure the TCP/IP stack with an IP address from the IANA-reserved Class B network 169.254.0.0 with the subnet mask 255.255.0.0.

To determine whether your system has APIPA enabled, go to a command prompt and type

ipconfig /all

Then, locate Autoconfiguration Enabled in the output. If this parameter is set to yes, APIPA is enabled. If you have an IP address in the range 169.254.xxx.xxx, APIPA is active.

To disable APIPA for selected interfaces on your system without disabling DHCP, run regedit and modify or add the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\Adapters registry subkey. Set the IPAutoconfigurationEnabled REG_DWORD entry to a hexadecimal value of 0. If your system has more than one network interface, you can disable APIPA on the computer. To disable APIPA for all the NICs, run regedit and go to the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces registry subkey. Modify or add a REG_DWORD value named IPAutoconfigurationEnabled, and set the value to a hex value of 0. After you modify one of these registry subkeys, you need to reboot your computer for the change to take effect.