Automatic Private Internet Protocol Addressing (APIPA) is a common alternative to the use of the Dynamic Host Configuration Protocol (DHCP) to request and retrieve an Internet Protocol (IP) address for a host. APIPA simplifies the assignment of IP address and subnet-mask configuration information to hosts in small networks. When APIPA is used, the operating system allows the assignment of a unique IP address to each station on a small local area network (LAN). This avoids the administrative overhead of running a DHCP server or manually setting IP configuration information.

APIPA is a procedure and set of guidelines for performing these configuration functions, not an actual protocol. It first appeared on the Windows platform in Windows 2000.

Determining an Interface's IP Address

To obtain configuration information such as address and network gateway for a network interface on the local computer, Windows Server 2003 does the following:

1. Contacts a DHCP server on the network and queries for configuration settings.
2. If no DHCP server responds, Windows Server 2003 checks for an alternate configuration that contains settings.
3. If it does not find an alternate configuration, the operating system uses APIPA to automatically configure its network settings, assigning an IP address that is unique to the LAN.

IP addresses assigned by Windows Server 2003 APIPA are within the range 169.254.0.1 through 169.254.255.254 inclusive, in accordance with specifications created by the Internet Assigned Numbers Authority (IANA). APIPA also sets the subnet mask on the network to 255.255.255.0.

For more information about client configuration settings, see DHCP Automatic Client Configuration.

Implications for the Network Application Developer

Awareness of the range of addresses used with APIPA is useful in troubleshooting. While APIPA is employed in many small environments that consist of 100 stations or fewer and no routers, it is normally not found in larger networks. If an IP address in this range does appear during testing on a large network, verify that the host in question is properly connected to the network. Also, while a home user can employ APIPA on their network, the external network interface connected to the Internet should not have an IP address in the APIPA range, unless a proxy or Network Address Translation (NAT) device is used for the Internet connection. According to IANA rules, the private address range APIPA uses is not for use on the public Internet.

Note   Appearance of IP addresses in the APIPA range on a network that normally assigns addresses using DHCP might indicate a connectivity issue or a DHCP server configuration problem.

Disabling APIPA

In Windows 2000, Windows XP, and Windows Server 2003, it is possible to disable APIPA for all adapters on the system by editing the registry.

To disable APIPA

1. From the Windows Start menu, choose Run, and then type regedit. The Registry Editor is displayed.
2. Navigate to the key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters.
3. From the Edit menu, choose New DWORD value.
4. Name the value IPAutoconfigurationEnabled and enter a value of 0.

See Also

Assigning TCP/IP Addresses, IP Features