APIPA, also known as Automatic Private IP Addressing, is a feature used in Windows operating systems. It comes into action only when DHCP (Dynamic Host Configuration Protocol) servers are available. When the DHCP client first comes on, it will try to establish a connection with the DHCP server in order to get an IP address. It is when this server is (or at a later point becomes) unavailable, that APIPA will kick in.

As the client is unable to connect with the server, APIPA will automatically try to configure itself with an IP address from an specially reserved range. (This reserved IP address range goes from 169.254.0.0 to 169.254.255.255).

After an IP address is obtained using APIPA, the client will then verify that their IP address is a unique one on the LAN. This is done utilizing ARP. According to Microsoft guidelines, APIPA will also check regularly to see whether the DHCP servers are available. If at any point it does become available, then the APIPA service will discontinue itself de-allocating the IP address it has obtained, and allow the DHCP server to allocate a dynamic IP address.

When APIPA takes over and your client has been assigned an IP address, the client will be able to communicate with other computers also on that LAN which have also been configured by APIPA, or failing that, which have been assigned a static IP address in the range mentioned earlier (so and IP address like 169.254.x.x, where “.x.x” are the identifiers unique to that particular computer).

However, if you are connected to the internet without a NAT, or even a proxy server, or you use routers, you might want to turn the APIPA feature off. For more information on this you can go to straight to the source at Microsoft’s Help and Support pages on “How to use automatic TCP/IP addressing without a DHCP server” at this address http://support.microsoft.com/kb/q220874/

All the information you need to on how to turn APIPA off, is contained here.

In essence the APIPA service is a fail-safe device put in place by Microsoft to make sure that everything runs smoothly. There is nothing that you need to do about it as it all happens in the background, and at speeds and times of which you are unaware of. In fact, by the time you detect that your DHCP server is unavailable, and you try to rectify the matter, APIPA would have already taken the proper steps necessary to keep your client running smoothly.

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