Full Form of APIPA:
Automatic Private Internet Protocol Addressing
APIPA Full Form is Automatic Private Internet Protocol Addressing. APIPA is a kind of feature available in operating systems (especially windows based OS such as Windows 98, Windows 2000, and Windows XP) that enable a personal computer to assign an IP address by itself when the DHCP (Dynamic Host Configuration Protocol) is not available. It serves as a backup operational mode for DHCP server and makes it convenient to manage small Local Area Networks. To connect to a network, each user or device in a LAN network is assigned a unique number called Internet Protocol (IP).
The IP address helps connect the device to the network. However, in case of multiple devices and devices moving in and out of the system, it becomes very difficult to manually configure the IP for each device with the network. Hence, a protocol called DHCP – Dynamic Host Configuration Protocol was developed. Here, the devices seeking connection to the network are automatically given the network configuration and can be assigned an IP address.
The DHCP is managed by DHCP servers that assign addresses for devices seeking the local network. However, when the DHCP servers are unavailable or are shut down, a server failover mechanism called Automatic Private Internet Protocol Addressing APIPA was developed by Microsoft. This was designed especially for Windows. In case of unavailability of the DHCP, the device automatically selects an IP address from a range of predefined addresses called APIPA.
The predefined addresses range from 169.254.0.0 – 169.254.255.255 (permitted by the IANA). Once the APIPA is automatically assigned, the device and communicate over TCP/IP with other devices on the LAN. To prevent that two devices do not assign themselves the same IP address in a network, Address Resolution Protocol (ARP) is present. The APIPA regularly checks for the presence of the DHCP server during operation. The APIPA automatically stops once DHCP server is available and running. Thus, APIPA acts like a self-configured IP address in the absence of a DHCP.