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DHCP Conflicts (Hopefully) Explained

January 6, 2017

If you've got a wi-fi network (who doesn't), at some point you've probably heard the terms Bridge Mode, DHCP, and router. As a matter of fact, one of the most common network problems I deal with involves all three: a DHCP conflict. But what does that mean, why is it a problem, and, most importantly, what can you do about it? 

 

I've put together this analogy that I hope will explain this in a way the average person can understand. Of course the thing about analogies is that they're by their very definition an analog—a close approximation—and as a result, they're not completely technically accurate. SysAdmins or other tech heads who read this are going to find several spots where they say "That's not right!" If you want a technical explanation you'll find them all over the web, but in my experience they're often only understandable by the people who already have a good foundation in the topic.

 

Think of a network like a neighborhood. Instead of places being defined by an address we would recognize, such as 1234 Main Street, Boulder, Colorado, 80303, a network uses an "IP address” which is a series of numbers, such as 192.168.0.1. The numbers in the IP address define the address much like a mailing address does. The first set of numbers (192) define the broad locality, similar to the state of Colorado in our example above. The second number (168) would be like the city. The third number (0) is like the zip code. The last number (1) is like the house number and street. 

 

The series of numbers gives information about where in the world the device is located. “Local” networks, such as you would find within a home or business, usually have an address range of either 192.168.0.# or 10.0.1.#; whereas on the larger internet a website would have an address like 17.172.224.47 (which belongs to Apple.com). Anything within 17.172.#.# is within Apple’s network neighborhood. (You can actually go to 17.172.224.47 in Safari and it’ll take you right to Apple’s website. We just use addresses like Apple.com because they’re easier for us humans to remember.)

 

Your network is being provided by a router, and your router functions as a real estate agent named DHCP. When a device moves into your network neighborhood it needs to move into into an empty house, which in this case is an IP address that isn’t being used. No two houses in a neighborhood can have the same address, and that's true in our network neighborhood as well; so only one network device can "live" at 192.168.0.63, for example. 

 

Your router generally lives at #.#.#.1. Everything after that, up to #.#.#.255, is available for other devices on the network. So if your router lives at 192.168.0.1, then your printer might live at 192.168.0.13, and your computer might live at 192.168.0.223. The numbers themselves don’t matter, as long as they’re not already in use, and as long as they’re in the same “zip code” (192.168.0.#). When a device first connects to your network, DHCP says “There’s an empty house at such and such; you live there now.”

 

So you've got a network and it's working great. DHCP is in charge of your neighborhood at 192.168.0.# and everything is going great; all the neighbors get along, and everyone knows where everyone lives. But then Comcast installs a new modem, and it came with its own DHCP that thinks it's in charge of the 10.0.0.# neighborhood. Now you have two real estate agents named DHCP in charge of two totally different neighborhoods. This problem is called dual DHCP. When a device connects to the network, it says “Hey, I need an address!” and one of the DHCPs, seemingly at random, says “There’s an empty house at ______, move in there.” Problems start.

 

So your Sonos might be living at 10.0.0.24, and your printer at 10.0.0.12, but your computer got moved into 192.168.0.3. Your computer says, “Hey, I need to talk to Sonos.” But DHCP is kind of an idiot—he doesn’t know anything about 10.0.0.#, as far as he knows only 192.168.0.# exists. He says “Sorry, can’t help you,” and your computer says “Printer not found."

 

A classic sign of dual DHCP is that rebooting one of your routers temporarily fixes the problem. When you reboot one of the routers, you temporarily kick everyone out of their neighborhood, let's say the one at 10.0.0.#. But they want to live somewhere, so they look for another real estate agent and found the other DHCP who promptly moved them into 192.168.0.#. Now they can all see each other again for the time being. But as devices go to sleep and wake up, or move around the house looking for strong wifi, when they reconnect to the network they get put into a random neighborhood and could end up back in 10.0.0.#. 

 

If you've confirmed that your problem is dual DHCP, what you need to do is put one of your routers into “bridge mode.” This is pretty much as it sounds—it builds a bridge from one neighborhood to the other, connecting them together, so that there’s only one big network neighborhood, all with the same address range, and all being controlled by a single DHCP.

 

So the key to fixing this common problem is to make sure only one router is providing DHCP and any others are in bridge mode (you typically want to leave the modem in charge of DHCP, so any other network routers like an Airport Extreme or Netgear base station are in bridge mode). How to do this varies by model, but instructions can usually be found online. Or, of course, you can call us to take care of it. But we're all about empowering our clients, so we hope you found this informative. Feel free to let us know if you still have any questions!

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