I get this question a lot: “Why am I getting this message on my computer? Another device on the network is using your computer’s IP address (192.168.0.14).”
Just about any computer nerd can tell you “because you have another computer on your network with the same IP address, dummy.” Well, sure you do, that is what the message says. But why are you getting that message and more importantly, how do you fix it?
DHCP, our computer nerdy acronym for Dynamic Host Configuration Protocol (if you haven’t noticed, we have TONS of computer nerdy acronyms so that you laymen folks don’t have a clue what we are talking about and have to pay us big bucks to fix nerdy stuff) is a way of automatically assigned TCP/IP addresses to devices on a network.
Get that? No?
Let me simplify it even more.
Say you are going to attend a Presidential Ball. Yea, with the head honcho, the guy we all love (or hate, depending on your party affiliation), the POTUS as the trendies now call him. You can be sure at a Presidential Ball, EVERYONE has a seat assignment. You don’t just show up to the ball and sit anywhere you want.
So how is that handled? Typically a coordinator will handle the seating assignments and pass those out as people show up (or in advance). It would be a HUGE embarrassment and breach of protocol for two people to have the same seating assignment, right?
DHCP is the “seating protocol” of your network. Every device on your network must have a unique “seat assignment” or TCP/IP address. And a DHCP server is your “seating coordinator”. It gives each device a unique address when it connects to your network. Even the public Internet has a global worldwide “seating coordinator”. Well, each major continent has a coordinator. In America it is ARIN – the American Registry for Internet Naming. They dole out public IP’s to Internet Service Providers (ISP’s) such as COX and AT&T. But that is on a much larger scale. We are going to keep this on a smaller scale – your office or home.
So How does DHCP Work?
So who is this DHCP server or coordinator and how do you manage it? Well, in the old days the person building the network physically assigned each IP address to each device. They would hard code it into the software. This worked fine in the world of mainframes and minicomputers. You bring in a new mainframe, your computer guy gave it a unique IP address, and it never needed to be changed. Just type it into the device once and you are done.
That worked fine until PC’s started showing up, especially laptop and portable computers. These would come and go and could be connected to various networks and different times. The computer user didn’t want to hand code the IP address for each network each time they moved the computer. So the techie network folks came up with a DHCP server concept.
A DHCP server sits on your network and listens for IP requests. A computer turns on, sends out a broadcast packet basically shouting “MY NAME IS AA-AH-B6-D4-67 and I NEED A SEAT ASSIGNMENT” or IP address. (Note: Techies like me may intermix the TCP/IP with the IP acronym. IP is a subset of TCP/IP – the back half. Often you will hear the term IP address. That is what we are talking about. IP address and TCP/IP address can be used interchangeably for the most part.)
The DHCP server hears this request and replies with a seat assignment and a reservation length. In other words, that seat assignment is good for the next 24 hours. You leave, come back, you get the same seat. If you are gone longer than the reserved time, you get a new seat. The devices IP address, or seat assignment, is good for the length of the reservation time, usually 24 hours but sometimes longer. Same goes for your computer devices.
How do I know if I have a DHCP Server?
You almost assuredly have a DHCP server on your network. Your ISP probably provided you with a wireless router, and most wireless routers have a DHCP server installed by default. And it is configured with default settings. But what happens if you have two wireless routers, both with a DHCP server set to default settings? That is like having two coordinator, both with the same seats to share, but without either of them knowing what the other is doing, or even knowing someone else is also giving out seating assignments.
What if I have more than one DHCP Server?
It is very easy to end up with multiple DHCP servers on your network. Do you think it would be a major problem if at the presidential ball there were two coordinators handing out seating assignments? And how about if they didn’t know about each other or what the other person was doing? Think that would be a disaster? Yep. Having two heads of states vying for the same seat could cause a war! Or at least be quite embarrassing.
Just like having two coordinators, having two DHCP servers on your network can be a BIG problem unless they know about each other and are synchronized. Each time a device pops up on your network and asks for an address, the first DHCP server that responds will give out an address. It could be the same address that the other server gave out 10 minutes ago. You can’t have two identical addresses on the same network. Things get screwed up really quickly.
First you gotta find out which device or devices are acting as a DHCP server. A LOT of devices can act as a DHCP servers – routers, switches, wireless access points, even other computers. Typically in a small network the culprit is having more than one router or wireless device. Once you find out what devices on your network is acting as a DHCP server, you gotta stop all the others from providing DHCP addresses except one DHCP server. Pick your “coordinator” and let them doing the coordinating. Turn the others off or at least shut them up from giving out IP addresses. Usually you will manage this DHCP function by logging into your device and going through the menus. Often this is a web browser you will use to log into the system, and will have to read your manual to figure out how to do this.
Once you are in your DHCP server, you typically have some control over how IP’s or “seats” are assigned. Want to sit the First Lady always to the right of the President? You create a “reservation” in your DHCP server. In other words, you tell your DHCP server “every time the first lady asks for a seat, sit her in the seat to the right of POTUS”. In your DHCP you tell it “each time this particular DEVICE – identified by its MAC address – asks for an IP, give it THIS EXACT IP. And don’t give it to anyone else.”
There are ways on large networks where you can coordinate having multiple DHCP servers so they share the workload and coordinate their address reservations, but that is beyond the scope of this article.
So how do I fix this duplicate IP problem, Mr. IT Smartypants?
The easy way is to call a knowledgeable networking person like me. But that isn’t cheap. Another way is to try and figure out why it is happening. In most cases on a small network it is because multiple devices on your network are acting as DHCP servers and not coordinating their actions. You gotta figure out what devices are causing this and either turn one of them off or stop it from acting as a DHCP server. You want ONE DEVICE acting as your DHCP server. In most smaller networks this is caused by having multiple wireless routers, or a wired router and a separate wireless router, or having a switch and a wireless router all acting as a DHCP server. You have to track that down, pick the device that is going to be your primary DHCP server, set it up the way you want, and turn the DHCP server function OFF on the other devices.
How do I stop this from happening in the future?
The easy way is to have only one device acting as both your firewall, wireless router, switch, etc. Don’t go out and buy a lot of wireless routers to extend the reach of your network. KISS – Keep It Simple Stupid – always is the best option.
But sometimes that is not an option. You need to have multiple switches, wired routers and/or wireless routers on your network. In that case, pick the device that is going to act as your DHCP server and make sure DHCP server is TURNED OFF on the other devices. If you have your ISP technician set up your network, be sure and let them know what is going on and if you have other devices on your network. Most technicians won’t know any more than the basics about DHCP so that might not do you any good, but make sure they keep things simple. Some know their stuff, many do not.
If you have a need to expand your network beyond a single router or wireless access point, and don’t have the IT skills to manage DHCP servers, then you probably need to call an IT pro. And pay the big bucks to get it done right.
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