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).
What is an automatic private IP address APIPA address?
An APIPA address is an automatically-assigned private IP address. It is typically used in small networks where a DHCP server is not present. APIPA addresses are in the range of 169.254.0.1 to 169.254.255.254. When a host on a small network can’t reach a DHCP server, it will automatically assign itself an APIPA address.
As discussed above, 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 re-register 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:
- Open Registry Editor (choose Start and then Run and then type regedit).
- 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).
- Right-click on the <AdapterName> key and choose New and then select DWORD Value.
- Enter IPAutoconfigurationEnabled to rename the DWORD value.
- Double-click the new value and ensure that 0 is entered as the value data.
- 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.