Question

How To Find DHCP Server On Network

Asked by: compsol1993

Hello,

We have a network composed of:
1 Linksys Router
2 Linksys Wireless Access Points (Not Routers)
2 16-port 10/100 switches

We recently needed to open a port on the router, and did not have the password so we reset the router and reconfigured.  

The issue is this; before we reset the router, we handled addresses through DHCP (on that router) handing out addresses 192.168.0.XXX.  The router's IP is 192.168.0.200.  

After we reset the router, it defaulted to an IP 192.168.1.1 and the DHCP scheme was 192.168.1.XXX.  We changed the router's IP to 192.168.0.200 and the DHCP scheme updated automatically in the router configuration.  

The problem is, DHCP is still being served on the 192.168.1.XXX scheme.  According to a client connected to the network, the DHCP server is 192.168.1.1.  If I pin 192.168.1.1 I do get a response.  I can also ping 192.168.0.200 which should be the address of the router.

Unfortunately I cannot power cycle or disconnect any network component until the weekend, since this is a doctor's office that needs maximum uptime.  To get everyone up and running, I setup static IP address on each computer.

I plan on going in tomorrow when the office is closed and troubleshoot further.  My first step will be to connect a laptop only to the router and see what IP I receive.  I suspect that the main router is not the culprit.  It does not make sense that I can ping both 192.168.0.200 and 192.168.1.1.  

When I attempt to go to the configuration pages:
http://192.168.0.200 brings me to my router's configuration page
http://192.168.1.1 says page not found
A port scan of 192.168.1.1 yields no open ports

Assuming the router is not the problem, something else on the network must also be handing out address via DHCP, and for some reason is taking precedence.  

Does anyone have any suggestions on this issue.  Am I correct in thinking some other device on the network must be causing the problem?  Is it possible that the router could exist at two IP address, both able to be pinged?  If it is another device, can you think of any better way to identify it than by simply pulling cables out of the switches one at a time?  I've tried a port scan, name lookup, nothing yielded a result.

Thanks for the advice.

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Asked On
2007-11-09 at 06:55:51ID22950103
Tags

dhcp

,

find

,

server

,

how

,

network

Topics

Networking Hardware

,

Miscellaneous Networking

,

Network Management

Participating Experts
5
Points
500
Comments
8

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Answers

 

by: hanckePosted on 2007-11-09 at 07:35:45ID: 20250173


  Your router is set to one address - the 0.200, but it is set up on its DHCP settings to hand out addresses of the 1.x range.

 Login to your router through the 0.200 page  and change the DHCP scope to your preferred 0.x range.

 

by: BrughPosted on 2007-11-09 at 07:40:34ID: 20250229

So wait,
Computer A gets an ip of 192.168.1.30 from DHCP and using that IP config you can ping both 192.168.1.x and 192.168.0.200?  Can you ping any other 192.168.0 addy?
Is your mask is correctly set at 255.255.255.0?
Do you have a windows Server running DHCP?  It could have authorized itself when you switched the IP on the router.  Check those.

Troubleshooting Step:
Plug a machine directly into the router and remove it from the rest of the network.  Run an ipconfig /release and an Ipconfig /renew.   Do you correctly get a 192.168.0.x address?  If no, the issue is with your router, verify that the DHCP server settigs are correct.  If yes, the issue is with another DHCP server on the network.

 - Brugh

 

by: JohnjcesPosted on 2007-11-09 at 08:09:30ID: 20250496

If no, you can do an ipconfig /all and it should reveal the IP of your DHCP server which may help you out as well.

John

 

by: chikenheadPosted on 2007-11-09 at 12:08:09ID: 20252546

Ipconfig /flushdns   then try pinging 192.168.1.1 you should no longer get a response.  Your computer caches ip to MAC address (arp resolution).  OR you can simply right click the ip connection from the system tray and select repair connection and it will also clear the cache for you.
Furthermore, check the date and time that the DHCP lease was issued.   Most likely it was issued prior to the changes you made.  DHCP leas times vary, but it should be noted that ip addresses do not immediately update, they stay assigned until the client requests a new one.  The cleint will generally request a new lease half way through the lease if the settings have changed.  For example if you have a one week lease period, a clent could recieve an address this morning, you could make a scope change this afternoon, and the change would not show up on the client for three more days--- assuming the clients DHCP is functioning properly.  Windows (all flavors 2000 XP 98)  sometimes has issues with releasing and renewing dhcp addresses.  In these cases a reeboot may be necessary.  However, selecting repair connection should solve this as well as it will release and renew the ip address.  If this does not resolve the issue, try to Reeboot the router.

 

by: compsol1993Posted on 2007-11-09 at 16:19:02ID: 20254111

Hancke:

Sorry if I was unclear, actually the DHCP range is set to 0.XXX

Brugh:

Well actually we have our server statically configured as 192.168.0.1.  I should have mentioned, that can ping both 192.168.1.1 and 192.168.0.200.  I'll check the mask, I hadn't thought of that. Other clients, who receive the DHCP in the 1.XXX scheme cannot ping 192.168.0.200.  All computers that obtain a lease can ping 1.1

Johnjces:

Actually thats how I determined 192.168.1.1 is handing out addresses.  

Cheikenhead:

I've tried repair several times using a client laptop.  The DHCP lease is not stored, as I brought in an external laptop that had never been on the network before.  This was after the problem started.  The least was a fresh lease and still showed this problem.

I initially rebooted the router several times, it did not help.

Thanks for all of your advice, if you have anything else to add based on this information, I'd appreciate it.

 

by: hanckePosted on 2007-11-12 at 05:55:54ID: 20263894


  Do you use a Nortel phone system?  I saw the same scenario on another small network and it turned out that the Nortel phone box - that also handled some other responsibilities - was handing out DHCP.   Just a direction to look.  

 

by: compsol1993Posted on 2007-11-13 at 05:40:16ID: 20271291

Thanks for your help everyone.  When I went to the location on Saturday, the problem disappeared.  I turned on every computer in the office, tested from every location, but the "phantom dhcp" was no longer running.  I will leave this post active for another week to see if the problem comes back, if not I will distribute points accordingly.  

 

by: OrcaKnightPosted on 2008-02-15 at 07:36:36ID: 20902626

Just a note: Because you have wireless routers, you might consider that someone else was piggy-backing on your network when you reset, and still had thier systems connected to yours, until the latest post. That would, in light of the lack of any other DHCP server in your known network, explain the sudden gain of a phantom, pingable device.

20120131-EE-VQP-002

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