APIPA - Automatic Private IP Addressing

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Automatic Private IP Addressing (APIPA) is a DHCP failover mechanism for local Internet Protocol version 4 (IPv4) networks supported by Microsoft Windows. With APIPA, DHCP clients can obtain IP addresses when DHCP servers are non-functional. APIPA exists in all modern versions of Windows including Windows 10.

How APIPA Works

Networks set up for dynamic addressing rely on a DHCP server to manage the pool of available local IP addresses.

Whenever 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 allocates an IP addresses from the private range 169.254.0.1 to 169.254.255.254. Using ARP, clients verify the chosen APIPA address is unique on the network before deciding to use it. Clients then continue checking back with the DHCP server at a periodic interval (usually 5 minutes) and update their addresses automatically when the DHCP server is again able to service requests.

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. In Windows utilities like ipconfig, this option is also called "Autoconfiguration."  The feature can be disabled by a computer administrator by editing the Windows Registry and setting the following key value to 0:

HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/TcpipParameters/IPAutoconfigurationEnabled

Network administrators (and savvy computer users) recognize these special addresses as failures in the DHCP process. They indicate network troubleshooting is required to identify and resolve the issue(s) 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 but still 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 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 must not attempt to manually assign addresses in the APIPA range else 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.

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