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. It invokes 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 or a network glitch interferes with the request, this process can fail.

When the DHCP process fails, Windows automatically assigns an IP address from the private range, which is to Using Address Resolution Protocol (ARP), clients verify that the chosen APIPA address is unique on the network before they use it. Clients then check 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 that has Windows 10 installed, for example, it waits for a few 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, and all reside on the same subnet.

APIPA Configuration

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:

Parameters\Interfaces\{network interface ID}\

If IPAutoconfiguration Enabled isn't listed, then it is by default set to 1. Instead, add a new REG_DWORD and set it to 0.

Failures in the DHCP process indicate network troubleshooting is needed to identify and resolve the issues that prevent 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 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 the network. 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, avoid using those addresses for any other purpose and instead limit networks to use the standard IP address ranges.

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