APIPA is another thing you will need to learn about when it comes to Network Fundamentals. We will divide all about APIPA into two parts. In this first part I will talk about APIPA in general and some of the functions you definitely need to know about, so stay concentrated!
APIPA stands for Automatic Private IP Addressing. What does it do? It is a Windows function, and it serves to provide DHCP auto configuration addressing. It assigns a class B IP address, we have already talked about. That means that it has the variety of client’s possibilities, from 169.254.0.0 to 169.254.255.255. Mostly it is used once the DHCP server is not available at the moment, but it can become available later. Then, the address of APIPA becomes replaced with some of the DHCP servers.
Let’s explain it in the example for your better understanding! A Windows Vista machine starts up. How long does it wait to find some of the DHCP servers? It needs only six seconds before it finds and assigns to some IP coming from APIPA’s range. Once that happens, it is continuing to search for a DHCP server. If we take a look at some of the previous versions of Windows, we can see that it took them for about three minutes to find a DHCP.
So, the point is that APIPA nowadays exists in all the versions of the modern Windows, of course, including Windows 10. It has the internet protocol 4 (IPv4).
What happens if the client’s device on Windows attempts to join some of the local networks? It happens that the APIPA immediately contacts the DHCP server with a purpose to request that IP address. But, what if the DHCP somehow stops functioning? Then, network glitch interface must interfere with all the requests that were sent. It is the same if some error enters the Windows. In these situations, it may seem that the whole process will fail.
But, by using the ARP, clients will know how to verify their APIPA address safely with a unique IP address together. The client is then able to check anytime needed the back server of DHCP. After checking it (it lasts for about five minutes), the user has the possibility to update the addresses automatically. The result is that the DHCP server will be able to service all the requests that previously failed.
It is also important to know that all of the APIPA addresses use the same subnet. It is always enabled by default on Windows. How does that happen? It will be set default if a client’s PC network interface was configurated for DHCP. This option is also known under a term ‘Auto configuration’. Of course, this feature can be only disabled by the computer’s administrator. He can easily register the Window’s setting and change the following key to zero.
All in all, that would be the basics of APIPA. In the next chapter, we will learn more about it and about the limitation of APIPA!