sundance-communications.com/forum Forums Computers & Programming Computers, Software, Accessories DHCP Conflicts

I hope this question doesn't sound to idiotic, but I am not really strong in networking issues, so here goes. My wife and I have separate offices in our home each with computers, my son has a computer and we have four or five IP phones connected back to our main office. We work from home a lot. It's a pretty straight-forward setup and usually works really well.

All of this stuff is connected to our cable modem via a router, then a 16 port switch. Some are connected straight to one of the router's four wired ports, the rest on the 16 port switch.

We usually keep our computers on all the time due to the fact that we have a whole-house battery backup system with 23 hours run time, plus they take so long to boot up!

Every now and then, my wife loses the ability to communicate over the Internet and has to shut down and restart, sometimes several times. Last time, it came up with a message indicating that another device is using her computer's IP address.

I am somewhat familiar with DHCP and this sounds like a classic case of DHCP conflicts. It's like another device is coming onto the network, like my son starting his computer, me tinkering around with the IP phones, etc. We have NO wireless IP; everything is 100% wired.

I checked and found that the modem is configured to be a DHCP server and so is the router. When we tried to disable this in the router, things went really bad to the extent that we had to default the router back to factory settings. I am starting to wonder if both the modem AND the router are functioning as DHCP servers, resulting in occasional conflicts. Am I on the right track here?

Someone told me that since the router is designed to keep the network isolated from anything that's ahead of it on the WAN port, the cable modem can't be issuing the IP's. I don't buy that because she can plug her computer directly into the cable modem and without changing any TCP/IP settings, it will work. Still, since her problem is so sporadic, it makes perfect sense to me that this could very much be happening.

Her computer is really the only device on the network that's on 100% of the time. My son shuts his down, but occasionally connects his Playstation's network adapter. I am forever tinkering with our IP phones, even rebooting the IP card in our host phone system at the office. That being said, there are frequently devices entering and leaving the LAN, each time resulting in new IP's being leased via DHCP. When we took her computer in to be checked at a REAL computer service center, they were able to connect to their LAN with absolutely NO problem each and every time.

I am really a phone system guy, not an IT technician. Any insight would be greatly appreciated.

It all depends on how you are setup. Do all your network devices have a static ip or are they set to automatic?

Not sure on the setup of IP phones but I would think they need a public IP to be routable.

If your wife can plug her system directly into the modem then her network settings on her system appears to be set to automatic. Which means it is getting an IP address from your provider when she attempts to connect to the internet-thus using their DHCP server. And when connected to the router ports or switch it is getting an IP address from the router which would have DHCP server option turned on.
Sounds like you should go to the router options under DHCP and set it so that the IP address does not expire-therefore it can not release it and assign it to another device. Causing your conflict.
I personally don't like using DHCP servers and I give each device a static IP, but that is not always possible. You can turn DHCP on for "other devices" and still give each computer a static IP to resolve those conflicts.
You can also type: ipconfig /renew at the command line to force your computer to get a new IP address.

Hope that helps if not let me know and we can get you fixed up correctly

Thanks, Crabb:

No, the IP phones don't need static IP's; only the system's IP card requires it, pretty much like all IP-enabled TDM phone systems. All of our devices are set to automatic IP (as in DHCP) and none of them require a static whatsoever.

I didn't think about the expiration option in the router, but it's worth a try. I still wonder though, couldn't the modem be dishing out IP's instead of the ISP's server? It is DEFINITELY set to function as a DHCP server when I look into it's configuration. So here we have the ISP, the modem AND the router all handing out IP's at the same time. I guess the root of my question here is that can these devices be doing this in unison? I mean here's my take on it:

A new device comes along and asks for an IP. Since two, possibly three DHCP servers are in the picture, is it possible that they all jump into action and send out IP's and the computer just grabs the first one that comes along, regardless of which device sent it? Is it possible that a request makes it through from the ISP or does the most local DHCP server (the router) have priority in this heirarchy.

In a nutshell, can these servers that are in a "chained" invironment push IP's through the devices that might be upstream or would the local server be able to prevent IP's from coming in through the WAN. Does that make sense?

I agree with you about using static IP's on all devices and I have considered it, but we take the IP phones out to other locations a lot, even on vacation so we really won't want to change them. I guess we could try using static IP's for the computers though. Maybe we can try that just to see. Thanks again for you help.

Ed I have never seen a modem that has a built in DHCP server but I am sure they exsist. So I am not sure how they interact, but in a typical network environment multiple DHCP servers can co-exsist but can also create problems. Some people say never run two and others have configured it with no problems.
I would turn that feature off on the modem and just use the one on the router.
Give your computers a static IP and let everything else use the DHCP server.
I like static because I share drives across the network and it makes it easier for me to keep track of. (personal preference)

Many of the current modems do have DHCP servers. As for lease times, if you log off, and then log back on, your MAC address is recognized by the DHCP server. If your old address is still available (which in your situation should always be the case) it gives you the same IP address again. I say this should alwasy be your case as if you are running a Class C private address (192.168.X.1-255) you have more addresses available to your DHCP server than you are ever going to use.

Something else you should look at is, are any of the computers set up for ICS (Internet Connection Sharing). If this is the case, then the computer set up for ICS will assume the IP Address of 192.168.0.1. It will also try to serve as a DHCP server. Oh, and I have seen a switch with a DHCP server as well.

As for the modem, I would shut off the DHCP. Then I would set the router up with a static IP address. I personally use DHCP on several applications but set network devices such as routers, printers, etc. to static addresses. Laptops always get set to DHCP as they tend to travel to other locations. If I set them to a Static Address and they go somewhere else they may be set to the wrong subnet or cause an IP conflict.

Crabb:

Thanks for babysitting me here. Yes, I have a modem that the cable company provided (Motorola) that has the ability to function as a DHCP server. I too was surprised at this. From what I can tell though, I can't change that function-these programming changes can't be made on the customer side. Maybe the cable company can access it from their side and disable it. I think it would be easier to get a cheeseburger at church, but it's worth a try by calling them. I will have to do it when I have a couple of hours to spare on the phone. These people.....well you know the rest.

MacGyver:

I am using an el-crappo switch, so I seriously doubt that it is even thinking about being a DHCP server. Still, that comment you made about ICS sparked my interest. Since we are in a mixed Microsoft environment (wife & son have XP; the guy who pays for it all <me> has 98 on the old family computer with a 75 pound monitor), I will need to address this on two different O/S. Are you able to point me in the right direction to disable ICS? That's starting to make sense to me, especially since I have never heard of it!

I am easily impressed and motivated when it comes to this stuff since I just know enough to be dangerous.

Did you self-install the modem, via an installation kit? If so, you should be able to use the CD to reset the username/password on the modem.

You don't mention the brand of router you are using. The Linksys routers are (or used to be) pretty dumb devices, in that if they reset for any reason, they lose their DHCP lease table. So, they'll happily start handing out IP addresses to new devices coming on to the network without checking to see if a device already has that IP address. The end result was exactly the condition you describe. Hopefully, that was fixed in the latest firmware, but I wouldn't count on it.

If you turn off DHCP in the router, can your devices still acquire IP addresses? That would be a decent test.

Seems more and more of the cable/DSL modems are including basic router functionality. Mine's an ancient Toshiba with no such features, but I did run into a Westell DSL modem that had a Verizon-provided CD installation scheme. Once the username and password were set up, we could login and disable everything.

The modem is a Motorola, recently installed to replace the previous Terayon. My wife had the cable company furnish and install them since it was a promotional free installation package. The Motorola was installed as a replacement because when they were out for a service visit, the cable guy said the other one was junk and they were having a lot of problems with them. We have still had this same problem, regardless of which modem we were using.

The router I am currently using is a Linksys. We had a Multitech Routfinder 550VPN, but on Tuesday, we had new carpet installed and had to disconnect it. It would no longer communicate via the WAN port after that. Since we are no longer using the VPN functionality, we just pulled out our old Linksys and it fired right up. We will probably keep it that way.

We turned off DHCP in the Linksys router and then found that no other devices could connect to it (duh). This led me to believe that it's definitely providing the DHCP leases and not the modem. I defaulted the Linksys back to factory settings with the reset button and it has been working fine with the basic settings.

Honestly, I can't say for sure if that Multi Tech unit wasn't the culprit the whole time. Maybe it was just going bad and finally kicked the bucket when it lost power. Since we have all of this stuff on the battery backup system for the house, I don't think it ever lost power since it was installed. We have never completely run the batteries down in the two years we had that router.

I am still convinced that the modem must be providing DHCP leasing since my wife has always been able to connect her PC directly to it, and without changing any settings in the PC, she just reboots it and it magically has an IP address. It can't be getting it anywhere else. In the setup menu for the modem, it definitely indicates that it IS set up to function as a DHCP server.

This being said, do I really even need a router any more? I don't know how much, if any firewall capability the modem has to offer, so I may have just answered my own question.

Ed,
Shouldn't the Lan side of the router be a different network segment to the WAN? just spitballing here but could the dhcp on the modem be giving the routers wan side a ip address and it be confilict with one the router gives to the lan side? If you can change the network address on the dhcp server of the router to another address like 192.168.1.xxx that might solve this issue if it is that type of conflict.

Kerry

When connected directly to the modem your wife's computer is getting its IP Address from your provider and not the modem. When connected directly to the modem, you can type at a DOS prompt ipconfig /all it should display its current IP-probably a public one it got from the providers DHCP server. Your router is probably functioning as a firewall using NAT and yes I would leave it in unless you have firewall turned on in XP but the 98 machine has no protection. Without a firewall it is very easy to hack a network. If you want some detailed help shoot me an E-mail and I can send you a setup.