Introduction

Now we’re going to be talking about APIPA. And this stands for automatic private IP addressing. Now every computer has to have an IP address to communicate on a network. And when a computer is set to automatically obtain an IP address, it gets the IP from a DHCP server.

What Happens When a DHCP Server is Unreachable?

But what happens if the computer can’t reach the server? So for example what happens if a DHCP server goes down or if the communication to the server is lost. Now if this happens, the computers that are running Microsoft Windows, the computers themselves will self-assign their own IP address and these IP addresses will be in this range starting with 169.254.0.1. This type of self-assigned IP addressing is what’s called automatic private IP addressing.

Subnet Mask and Communication on the Local Network

And in addition, they were also assigned themselves a 16-bit subnet mask. Now the reason why computers do this is that even though they cannot get an IP address from a DHCP server, they will still be able to communicate with other computers on the same local network, or to be specific, on the same subnet that also have self-assigned IP addresses.

Limitations of APIPA

However, the computers will not be able to access the internet or access other devices on a different subnet with this type of IP address. So if you’re on a computer and you suspect that there is a networking issue and you can’t access the internet, just open up a command prompt and type in ipconfig and then check your configuration. And as you can see, this computer has configured itself with an automatic private IP address next to the category of auto configuration IPv4 address.

Example Scenario

So you can see the IP address of 169.254.47.4. So step by step. Let’s say in this office we have three computers and they’re all connected to the switch that’s connected to the DHCP server. And let’s say for example that the DHCP server cannot be reached.

Whether it’s due to a malfunction or disconnected cable or maybe even the server caught on fire? So when we turn on these computers, they are going to ask the DHCP server for an IP address. However since the server cannot be reached because it’s on fire, the computers are going to have to self-assign their own IP address from this range of IP addresses. But IP addresses have to be unique for each computer.

Ensuring Unique IP Addresses

So in order to ensure that the computers don’t pick the same IP address, the computers will broadcast an ARP message on the network to make sure that no other computer picks the same IP address. Then once they have all their IP addresses they will then be able to communicate with each other on the same subnet.

Reconnecting to the DHCP Server

Now every few minutes the computers will check to see if they can contact a DHCP server, hopefully one that’s not on fire, and if it detects one, the computer will replace the self-assigned IP address with the one obtained from the DHCP server.