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Why are IP's being assigned in DHCP that are not from my Scope

Posted on 2012-08-28
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Last Modified: 2012-08-28
Hello,

I have 1 DHCP server that we have been successfully using for the 3 plus years.  The Server is a Windows 2003 Server.  This Server acts as :

1.  Active Directory Server.
2.  DHCP Server.
3.  DNS Server.
4.  Backup Server.

The DHCP scope is (Starting) 10.220.21.55 - 10.220.21.254 (Ending).  The Server's IP address is 192.168.1.X; but, the scope that is gives out is 10.220.21.X/24 .  This problem is only with PC's on the DHCP Vlan.

One may wish to referece another separate Experts question as well

http://www.experts-exchange.com/Networking/Protocols/DHCP/Q_27844071.html 

Our Windows Domain Client PC's receive DHCP on a specific VLan(DHCP relay is enabled) and again it has been working fine for 3 - Plus years.  But recently some computers on the DHCP VLan are not connecting into the network.  When I check to see what is going on with the "rouge" pc's I notice that they have a different IP address.

IP:             192.168.1.X
Sub:           255.255.255.0
Def. Gtwy: 192.168.1.1

Which is weird because the 192.168 - Scheme that the DHCP server uses is different.  The IP  Scheme (for what we call VLan-1) is:

IP:              192.168.1.X
Sub:           255.255.254.0
Def. Gtwy: 192.168.1.25

So, to correct the 'Rouge' pc's that are not getting on our network and are on the DHCP VLan, I just open a command prompt and type

1.  ipconfig <enter>
     a.  I see the incorrect ip-scheme.

2.  ipconfig /release <enter>
     a.  to zero out the ip address info.

3.  ipconfig /renew <enter>
     a.  to pull a new (and correct) ip address.
     b.  10.220.21.X

4.  problem solved.


This is happening to more and more users, if they turn on their PC's with time.  If the users do not restart they are fine.  Its an easy fix; but, it is annoying?

My 2 questions are:

1.  Why is this happening.

2.  What can I do to stop this from happening?

I thought that someone connected a home router into a DHCP VLan'd network jack.  And this other router is competing with the corporate network to give out IP Addresses.
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Question by:Pkafkas
5 Comments
 
LVL 37

Accepted Solution

by:
Neil Russell earned 500 total points
ID: 38342390
I would start looking for a rogue device handing out DHCP. Maybe a wireless access point or a new router of some description. 192.168.1.1 is indicative of a default DHCP server handing out addresses somewhere.
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LVL 23

Assisted Solution

by:Suliman Abu Kharroub
Suliman Abu Kharroub earned 500 total points
ID: 38342427
While the clicnt is using the incorrect ip config, run

ipconfig /all

it will show the DHCP server which offered the IP address to that client.
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LVL 6

Assisted Solution

by:airborne1128
airborne1128 earned 500 total points
ID: 38342461
Sounds like someone on your network has plugged in a switch or router of their own.  A home router usually comes with DHCP enabled and issues addresses begining with 192.168.1.x.
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LVL 2

Assisted Solution

by:DerekFG
DerekFG earned 500 total points
ID: 38342659
Sounds to me like someone decided they wanted to hook up a wireless router by themselves.  I've seen this so many times, it's annoying beyond belief.

Sulimanw has the right idea, see what the DHCP server is / default gateway and track that puppy down.
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LVL 1

Author Comment

by:Pkafkas
ID: 38343104
My suspisions and everyone else's in this thread are correct.  It was a rouge DHCP router that someone clied teh reset buttoon and becasue of that it collected an IP address with DHCP and it gave out IP Addresses with DHCP (with no password).

That explains this case and another question that I asked.  http://www.experts-exchange.com/Networking/Protocols/DHCP/Q_27844071.html
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