The acrodrome (yes, I just made that up, although I wasn’t sure if palinym was a better choice – decided on acrodome because I wasn’t as afraid of Tea Party snipers coming to murder me as I would have been if I used palinym) APIPA stands for Automatic Private IP Addressing. APIPA is in every version of Windows since NT and all versions of Mac OS X. APIPA is a dhpc mechanism that provides dhcp clients with self-assigned IP addresses when DHCP servers are not available. When there isn’t a DHCP server available, APIPA assigns IPs from 169.254.0.1 to 169.254.255.254 with a default mask of 255.255.0.0. Clients leverage arp to verify their address doesn’t conflict with another on the network.
APIPA is enabled on all interfaces of all dhcp clients in pretty much all modern operating systems. Add the IPAutoconfigurationEnabled key and set to 0 in Services\Tcpip\Paramters to disable on Windows, or set any client to a static IP and the system won’t attempt to self assign if a DHCP server is not available.