Windows uses Automatic Private IP Addressing (APIPA) when it is unable to obtain an IP address using DHCP. Typically, a DHCP server provides each computer with an IP address. However, sometimes Windows is unable to obtain an IP address from the DHCP server for a variety of reasons. Thus, when Windows detect a network connection, and it does not have a pre-assigned IP address, and it is unable to connect to the DHCP server, it will assign itself an APIPA address.

APIPA addresses fall in the range of 169.254.0.1 to 169.254.255.254. All devices use the default network mask 255.255.0.0 and all reside on the same subnet. When using APIPA, it is possible to ping other computers on the subnet, but it is not possible to ping computers that are external to the subnet.

Client applications can verify their address is unique on the LAN using Address Resolution Protocol (ARP). When Windows is finally able to communicate with the DHCP server, it will replace the APIPA address with the newly assigned IP address that Windows obtained from the DHCP server. APIPA is enabled by default on all DHCP clients in Windows. It is possible to modify the Windows Registry to disable APIPA.

If a computer has an APIPA address, it is unlikely that OPC will work as well. This is because the presence of an APIPA address usually indicates some kind of a network problem.

See also: ARP, DHCP, IP Address