Automatic Private Internet Protocol Addressing
Automatic Private Internet Protocol Addressing (APIPA) is a capability within Microsoft Windows that is on by default and has to be explicitly turned off if the user does not want it. APIPA is essentially a DHCP fail-over mechanism.
In APIPA, when a Windows system cannot locate a DHCP server, the local system allocates itself a private IP address in the range 169.254.0.1 to 169.254.255.254. Because it is critical that no two systems use the same IP address, the client has to have a way to verify that it is the only system in the local network using that particular address. It does so using ARP. The client simply issues an ARP message looking for its own, newly assigned IP address. If it gets a response, this means another system is using that address. The local client therefore allocates a different IP address and repeats the process. It will continue to do this until it finds an address no other system is using.
When APIPA is being used, the Windows client will periodically check for the presence of a DHCP server, and revert to using DHCP when it becomes available. The default interval for this period check is 5 minutes.
APIPA requires all devices to use the same default subnet mask of 255.255.0.0. The systems must all reside on the same network segment. APIPA cannot provide information about local routers or about DNS servers.
APIPA is intended for small business or residential network environments, usually less than 25 clients, that are not routed or connected to the Internet.
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