APIPA – Automatic Private IP Addressing

If you work with computers for a living or even do your own computer repair and configuration at home you may have heard of a term called APIPA or Automatic Private IP Addressing. If you are familiar with networks you know that computers need an IP address to communicate with each other. This is how your network in the office works and also how the internet works. An IP address looks something like and will vary depending on the network configuration. Each computer on the network has to have a unique IP address to communicate.

When a computer doesn’t have an IP address then it won’t be able to do things such as log into a domain, print to network printers or get files off of a file server. Computers are assigned IP addresses manually (static IP addressing) or automatically (dynamic IP addressing). Computers that get their IP addresses automatically need a source to provide those addresses. Usually this comes in the form of a DHCP server which has the main function of providing IP addresses to network clients. For many home users you usually get your IP address from your ISP for dialup and from your DSL/Cable modem for broadband internet connections. If you have a DSL/Cable router then it most likely provides your computer’s IP address. These IP addresses are leased to the computer for a set amount of time and then they need to be renewed or else they will expire.

Your computer will be set to either have a static (manual) or dynamic (automatic) IP address in its configuration. If its set to get its IP address automatically and there is no DHCP server or device on the network that can give it an IP address then it will give itself an APIPA address. This will usually be in the form of 169.254.x.x where x can be a variety of different numbers. The purpose of an APIPA address is that when the computer’s lease of its IP address expires then it will have at least some type of address that can be used for network communication. The computer will only be able to communicate with other computers that have a 169.254.x.x address which they will receive when their lease expires. If other devices such as servers and printers have static IP address then the APIPA client will not be able to communicate with them since they will be on a different IP subnet.

When you are troubleshooting network or internet connectivity it’s always a good idea to check your IP settings and make sure you have a valid IP address. If you see an APIPA address then you know to start looking at your DHCP source when it comes to troubleshooting.

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