APIPA is Automatic Private IP Addressing. It is a Microsoft’s terminology for the address autoconfiguration as it allows a Local Area Network (LAN) computer to give itself a unique IP address when Dynamic Host Configuration Protocol (DHCP) is unavailable.
With APIPA, DHCP clients can obtain IP addresses when DHCP servers are non – functional. APIPA exists in all modern versions of windows except Windows NT.It is enabled on all DHCP clients in Windows unless the computer’s Registry is modified to disable it.APIPA can also be enabled on individual network adaptors. It is also called DHCP failover mechanism (for local networks) or Auto-IP or AutoNet.
When a DHCP server fails, APIPA allocates IP addresses in the private range 169.254.0.1 to 169.254.255.255.Clients verify their address is unique on the network using ARP.When the DHCP server is again available to service request, clients update their IP addresses automatically.All devices use the default network mask 255.255.0.0 and all reside on the same subnets in APIPA.
APIPA for IPV4 and IPV6 addresses :-
Stateless address autoconfiguration procedures use two protocols for assigning IP addresses to machines connected to the internet: Internet Protocol version 4(IPv4) and Internet Protocol version 6(IPv6).These protocols are used when there is no DHCP server or other automatic methods for assigning unique IP addresses.IPv4 link-local addressing uses a block of addresses reserved by the Internet Engineering Task Force(IETF),with block range 169.254.0.1 to 169.254.255.255.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 algorithm ensures uniqueness of the IP address