Solved

IP Conflict

Posted on 2013-12-17
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Last Modified: 2014-01-17
A number of computer in my office has suddenly started getting IP conflict errors.

All these machines have static IPs which I know are not being used by any other PCs.

If I click diagnose then it says that DHCP is not enabled, I don't want DHCP enabled as they have static IPs.

If I do switch it to the DHCP then they get internet connection but it does not resolve the domain, it just says 'unidentified network'. This has not been an issue before.

This problem started with one user the other day and now 'seems' to be spreading to more people in the office.

Please note the IP settings on each PC are definitely correct.

Any ideas would be greatly appreciated?
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Question by:Ben_Locke
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by:Prashant Shrivastava
Prashant Shrivastava earned 100 total points
ID: 39723563
This clearly means that someone enabled DHCP services in your network but DHCP server is not passing on the corrdct DNS information thats why you are getting connection but not able to resolve Domain Names.

Suggest you to enable DHCP and check which server is announcing it as DHCP server and check the server, sometime this happens.

Regards,
Prashant.
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by:Prashant Shrivastava
ID: 39723597
for More information please find this link : this will help you with that - http://blogs.technet.com/b/teamdhcp/archive/2009/07/03/rogue-dhcp-server-detection.aspx
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by:omarfarid
omarfarid earned 100 total points
ID: 39723605
Do you have other DHCP servers in the network with the same IP range?

This could be a possible cause of the problem.

Have you done any changes or introduced new servers or devices in the network?
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Author Comment

by:Ben_Locke
ID: 39723633
Thanks for the responses guys, we only have one DHCP server, is the DHCP issue going to also cause the issue with static IPs?

No we haven't added any new devices.
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by:omarfarid
ID: 39723655
Have you experienced any DHCP crash or restart? If you do a DHCP lease renew (from command line run ipconfig /renew) on the client do you still see the problem on the client?
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Author Comment

by:Ben_Locke
ID: 39723685
Omarfarid, I tried that with no success.

I have installed wireshark on the server and captured some data, could you help me with what I need to look for to give me a clue as to what is going on?
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by:omarfarid
ID: 39723712
I am not familiar with that. Are the static IPs excluded from the IP range on the DHCP server? if not then your DHCP server is allocating them to other systems.

You may reserve these IPs on your DHCP server and assign them by DHCP server based on the MAC addresses of your system.
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Author Comment

by:Ben_Locke
ID: 39723726
The static ips are in the range 192.168.3.1-219 and the dhcp is 192.168.3.220-250.

When i try and use the DHCP it gives me an IP address 172.27.128.34.

Also in the lease view on the DHCP server every address is a BAD_ADDRESS.
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by:Surrano
Surrano earned 100 total points
ID: 39723923
Have you tried "arp -a" on any of your clients and servers? it may reveal the conflicting addresses.

If DHCP range is set to 192.168.something and it gives you 172.27.something then
it is either not happy with the range configuration (e.g. some routers don't accept any ranges other than 2-254) or
there's another dhcp server serving you instead.

Double check the DHCP logs; maybe provide a (disclosable) sample and we'll see if it helps.
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Author Comment

by:Ben_Locke
ID: 39723991
Thanks for all your suggestions.

I have discovered that it is a Netgear GS748TS switch.

I realise the setup isn't ideal but i have a Ethernet cable connecting that to the main network switches which are GS700TS.

This solution has worked fine in the past as I have two other switches connected like that without any issues.

How ever with this one, if I unplug it everything goes back to normal. As soon as I plug it back in it starts again.

Ideas?
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Expert Comment

by:Surrano
ID: 39724037
So which switch the clients are connected to and which switch provides the (wrong) DHCP addresses? It'd be nice to see the switch configurations...

also please check whether you properly follow the guidelines explained here:
http://kb.netgear.com/app/answers/detail/a_id/136
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by:omarfarid
ID: 39724297
So the other switch was also working as DHCP server?
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by:Fred Marshall
Fred Marshall earned 100 total points
ID: 39724705
I don't see that the Netgear switch can act as a DHCP server from the manual.  
The issue remains that there is another DHCP server somewhere on the network, no?

So maybe there's another such server on the network.    A rogue router.
For example, Joe brings in a Linksys wireless router from home and plugs it LAN-to-LAN into your network and uses one of the LAN ports on this for his office computer - using the router as a switch.  He sets the IP address for the Linksys in your subnet range and leaves DHCP turned on.  Now he has an access point for his phone!
This is happening often enough these days!
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Author Comment

by:Ben_Locke
ID: 39726027
The switches are not acting as the DHCP server, we have a DC that does the DHCP server role also.

This morning i have tried to stack this problem switch correctly with a HDMI cable but cannot get it work with my other 2.
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by:ChiefIT
ChiefIT earned 100 total points
ID: 39748600
If you enabled DHCP on the server and things started to get a 172... address and all leases are jacked up, It means that you have two nics on the server connected to your network. One trying to act as a DHCP server and communicate on a bad subnet, the other trying to support the real network.

Tell us more about your multiple nics on the server. IT CAN BE DONE with explicit config settings.  For now, I would disable the second nic until it's configured correctly.
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Accepted Solution

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Ben_Locke earned 0 total points
ID: 39776028
Turns out someone had plugged a network cable in from another network and this was then picking up that networks DHCP, removed the cable and all the problems went away.

Will spread the points out, thanks for all the suggestions.
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Author Closing Comment

by:Ben_Locke
ID: 39788065
My comment contains the fix for the problem
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