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Why is fixed ip changing to DHCP

Posted on 2013-11-09
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Last Modified: 2013-11-17
I have an environment that has 10 machines, each with fixed ip address.  Recently the network settings have been getting changed back to DHCP, supposedly on their own.

What can make the fixed ip settings get set back to DHCP.  I wasn't aware that could happen.

The operating system is both XP and Win 7
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Question by:c7c4c7
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by:John Hurst
ID: 39636124
There are General and Alternate settings for DHCP. Did you change both?

Did you set for Fixed IP? or just enter the IP address?

Is your fixed IP in the DHCP range at the DHCP source?  It should be outside the DHCP scope.

.... Thinkpads_User
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by:c7c4c7
ID: 39636598
There are General and Alternate settings for DHCP. Did you change both?
General only Alternate is for use with mobile devices. These are all desktop machines.

Did you set for Fixed IP? or just enter the IP address?
I entered in the Use the Following field in the adapter properties sheet.  I cannot find anything for a field titled Fixed IP

Is your fixed IP in the DHCP range at the DHCP source?  It should be outside the DHCP scope.
Outside the scope
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by:John Hurst
ID: 39636928
Thanks for the update. Please allow me to clarify a couple of things.

1. Alternate and General:  I have those tabs on both my  Desktop (Windows 7) and my Laptop (Windows 8.1). So that is why I asked.

2. Fixed IP:   You need to set IP, Subnet Mask, Gateway and DNS. In the settings window, there is a radio button for Fixed IP and Fixed DNS.  I was just asking if you had double checked these settings. See below:

DHCP-Settings
3. Scope:  Thanks for the update.

This is happening on 10 machines with different operating systems.

(a) Do you have IPv6 enabled?  You might try disabling IPv6 on one or two machines to see if that makes a difference. I have had IPv6 installed for years and it makes no difference to me, but disabling it has helped some people.

(b) What is your source of DHCP?  If router, have you upgraded the firmware in the router. You might even consider setting the router back to factory defaults and setting it up again.

If you have a Server providing DHCP services, make sure the Server is fully updated.

(c) Viruses:  This is what viruses do. Have you had a virus outbreak?  You should do a full scan on a couple of machines.

I use fixed IP addresses for my printer and a few other devices in my home office network and the devices never revert to DHCP. I have a fixed IP at one client and the computer never reverts to DHCP.

.... Thinkpads_User
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by:c7c4c7
ID: 39637278
2. Fixed IP:   All fields have been input.
And this is the root of the problem.  After the machine is used for days/ weeks/months the radio button for Obtain an IP address automatically gets filled in and the ip address is naturally changed to something in the DHCP range.

Everyone claims there is not human interaction to cause it but I am doubtful as I have never seen this modified once it gets filled in.  I am looking into creating a script to text me when the ip address changes and then change it back.  


(a) Do you have IPv6 enabled?  You might try disabling IPv6 on one or two machines to see if that makes a difference. I have had IPv6 installed for years and it makes no difference to me, but disabling it has helped some people.

-->   I think IPV6 is enabled by default on Win 7, I'll try this on a machine and see if it makes a difference, however it would not explain  why it happens on XP

(b) What is your source of DHCP?  If router, have you upgraded the firmware in the router. You might even consider setting the router back to factory defaults and setting it up again.

If you have a Server providing DHCP services, make sure the Server is fully updated.

--> The router served as DHCP host

(c) Viruses:  This is what viruses do. Have you had a virus outbreak?  You should do a full scan on a couple of machines.

I use fixed IP addresses for my printer and a few other devices in my home office network and the devices never revert to DHCP. I have a fixed IP at one client and the computer never reverts to DHCP.

--> I've used Malwarebytes as well as Norton to scan at least 2 machines, nothing found as far as viruses, there are the normal PUP's from Malwarebytes
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John Hurst earned 500 total points
ID: 39637448
You said that a router is the source of DHCP so check its firmware and consider a factory reset.

Since you are ruling out all causes and since this does not happen by itself, maybe you really do have some loose nuts behind the keyboards.

There is a cause, and if not router, not viruses, not settings and not whatever else, then someone is fiddling with the machines.

.... Thinkpads_User
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Expert Comment

by:Rob Williams
ID: 39638066
There was a known issue with Broadcom NIC's doing this a while back.  Are they all Broadcoms?   If so have you installed the latest Broadcom driver (not Windows)?
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by:FutureTechSysDOTcom
ID: 39646032
There is an easy fix for this - get the MAC ID for the machines, and put the MAC ID's in the fixed IP fields in your router.  Any decent $99 router should have this ability, and then, whether its fixed or static on the machine, it's going to grab the same IP address.

Or, you can do the same thing with the DHCP server if you have a server on your network.

Regards,
Chris M.
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by:John Hurst
ID: 39654776
@c7c4c7 - Thanks and I was happy to help you.

... Thinkpads_User
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