Automatic Private IP Addressing (APIPA) in Windows Server 2008



The Client/Server service has been updated in Windows 2000 clients and higher, enabling it to automatically assign itself an IP address if no server is available; it does so through a process called Automatic Private IP Addressing (APIPA). APIPA clients automatically assign themselves an IP address in the 169.254.0.0/16 range in this situation, which allows them to have basic TCP/IP connectivity in small networks.

APIPA might be problematic in larger networks because it forces clients to assign themselves addresses in a range that is normally not part of a local company subnet. If a DHCP server is down, clients that are attempting to renew a lease with the server will fail and automatically assign themselves an APIPA address. When the server comes back online, they will not immediately reregister themselves and will effectively be cut off from the network. Subsequently, Microsoft supplies a Registry key that will disable APIPA in this situation. The key to be created is

HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Param eters\Interfaces\<AdapterName>\_IPAutoconfiguratio nEnabled:REG_DWORD=0

You can create this key by following these steps on the client:

1. Open Registry Editor (choose Start and then Run and then type regedit).

2. Navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Servic es\Tcpip\Parameter\_Interfaces\<AdapterName> (where AdapterName is the hexadecimal representation of the network adapter in question).

3. Right-click on the <AdapterName> key and choose New and then select DWORD Value.

4. Enter IPAutoconfigurationEnabled to rename the DWORD value.

5. Double-click the new value and ensure that 0 is entered as the value data.

6. Click OK and close the Registry Editor.

To validate that APIPA is disabled, an administrator should run IPCONFIG /ALL from the command prompt and then check that the Autoconfiguration Enabled option is set to NO.

NOTE: APIPA can also be effectively disabled in Windows XP clients through an alternate IP configuration, which allows for the designation of a static IP address if DHCP is unavailable.