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Automatic Private IP Addressing (APIPA) is Microsoft’s terminology for address autoconfiguration in the Windows 98, ME, 2000 and XP OSs. APIPA allows a local area network (LAN) computer to give itself a unique IP address when Dynamic Host Configuration Protocol (DHCP) is unavailable.
APIPA is sometimes known as auto-IP.
Networked computers are differentiated by individually assigned IP addresses. The address is known as a link-local address, which is an IP address used for local communications with other LAN computers.
Stateless address autoconfiguration procedures use two protocols for assigning IP addresses to machines connected to the Internet: Internet Protocol Version 4 (IPv4) and Version 6 (IPv6). These protocols are used when there is no DHCP server or other automatic method for assigning unique IP addresses. Automatic IP addressing also may be used upon failure of a server with an IP address assignment function.
IPv4 link-local addressing uses a block of addresses reserved by the Internet Engineering Task Force (IETF). This address block range is 169.254.1.0 through 169.254.254.255.
The IPv6 protocol requires that an OS assign a link-local address to network interfaces and use the prefix “fe80::/10.” It also uses more than one IPv6 address for each machine’s network interface. A media access control (MAC) address-based methodology and duplicate address algorithms ensure IP address uniqueness.