what is APIPA ( Automatic Private IP Addressing )

Automatic Private IP Addressing

the 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. 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. 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. 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.  

Automatic Private IP Addressing

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. 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. 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. 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.