APIPA—Automatic Private IP Addressing

APIPA serves as a fail-safe during DHCP problems

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Automatic Private IP Addressing (APIPA) is a DHCP fail-safe that protects a computer system from failure by invoking a standby mechanism for local Internet Protocol version 4 (IPv4) networks supported by Microsoft Windows. With APIPA, DHCP clients can obtain IP addresses even when DHCP servers are not functional. APIPA exists in all modern versions of Windows, including Windows 10.

How APIPA Works

Networks that are set up for dynamic addressing rely on a DHCP server to manage the pool of available local IP addresses. When a Windows client device attempts to join the local network, it contacts the DHCP server to request its IP address. If the DHCP server stops functioning, a network glitch interferes with the request, or some issue occurs on the Windows device, this process can fail.

When the DHCP process fails, Windows automatically assigns an IP address from the private range, which is 169.254.0.1 to 169.254.255.254. Using Address Resolution Protocol (ARP), clients verify that the chosen APIPA address is unique on the network before they use it. Clients then check back with the DHCP server at periodic intervals—usually every five minutes—and update their addresses automatically when the DHCP server is able to service requests.

When you start a computer running Windows Vista, for example, it waits for only six seconds for a DHCP server before using an IP from the APIPA range. Earlier versions of Windows look for a DHCP server for as long as three minutes.

All APIPA devices use the default network mask 255.255.0.0, and all reside on the same subnet.

APIPA is enabled by default in Windows whenever the PC network interface is configured for DHCP. This option is called autoconfiguration in Windows utilities such as ipconfig. A computer administrator can disable the feature by editing the Windows Registry and setting the following key value to 0:

HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/TcpipParameters/IPAutoconfigurationEnabled

Network administrators and experienced computer users recognize that failures in the DHCP process indicate network troubleshooting is needed to identify and resolve the issues that are preventing DHCP from working properly.

Limitations of APIPA

APIPA addresses do not fall into any of the private IP address ranges defined by the Internet Protocol standard and are restricted for use on local networks only. Like private IP addresses, ping tests or any other connection requests from the internet and other outside networks cannot be made to APIPA devices directly.

APIPA-configured devices can communicate with peer devices on their local network but cannot communicate outside of it. While APIPA provides Windows clients a usable IP address, it does not provide the client with nameserver (DNS or WINS) and network gateway addresses as DHCP does.

Local networks should not attempt to manually assign addresses in the APIPA range because IP address conflicts will result. To maintain the benefit APIPA has of indicating DHCP failures, administrators should avoid using those addresses for any other purpose and instead limit their networks to use the standard IP address ranges.