APIPA – CompTIA Network+ N10-006 – 1.8

| April 3, 2015 | 0 Comments


The APIPA process ensures that your computer will be able to communicate on a local subnet, even when a DHCP server is not available. In this video, you’ll learn about the details of APIPA and how to determine if your computer has been assigned an APIPA address.

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In IP version 4, you either need to manually configure an IP address on your workstation or you need to have one automatically assigned by a DHCP server. But what if you’ve not assigned one on your workstation and for some reason you’re not able to communicate to a DHCP server? Fortunately, there is a middle ground and this middle ground is called an Automatic Private IP Address or a APIPA. This is what we call a link-local address. That means that it allows you to communicate to anybody who might be on the same subnet as you who has also been given an APIPA address. But it doesn’t allow you to route outside of that subnet.

So if you were assigned an APIPA address, you would not be able to communicate out to the internet for example. There is a range of addresses you will be given as an APIPA address. The IETF’s reserved 169.254.0.1 through 169.254.255.254. And in that address range, the first block and the last block of 256 addresses have been set aside as reserved. That means that if you do receive an APIPA address it will be somewhere between 169.254.1.0 through 169.254.254.255.

We often refer to this as the 169 address, because when somebody receives a 169.254, you know that they’ve gotten in APIPA address and they must be having problems communicating to their DHCP server. If you don’t have a statically assigned address and you can’t talk to a DHCP server, this APIPA address will effectively be random. You’ll be given an address to use by your operating system, and your operating system will check on the network using ARP to determine if there is another device on the network who might have already been given this same address. You don’t want two devices on the network with the same IP address. So we rely on ARP to make sure there are no duplicates on the network.

If you’re in Windows and you’ve been assigned an APIPA address, you’ll see it when you look at the network configuration of your device. This is an Intel Pro 1,000 MT Desktop Adapter is the Mac address of this device. And if we move down here, you’ll see auto configuration IPv4 address. So Windows even tells you this was an automatically assigned IP address by the operating system, and it’s 169.254. We already know it’s automated. And it’s 228.109, happens to be the full IP address. with a subnet mask of 255.255.0.0

We saw the 169.254. We knew it was an APIPA address. And just to confirm that, Windows even told us, this is the auto configuration address that’s been assigned to this device. If you’re on a machine and you were expecting a DHCP address and you end up getting an APIPA address, there’s some problem with your device communicating out onto the network and to the DHCP server. So you might have a bad cable. You may not be connected properly to the wireless network. There may be a routing problem in your network or the DHCP server maybe down. But all of those situations can cause a breakdown in communication. In that reason, you may be assigned an APIPA address.

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Category: CompTIA Network+ N10-006

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