IP address conflict on DHCP network

Posted on 2004-11-19
Last Modified: 2012-06-27
We have a small network in our office.  A one-port DSL router provides internal IP addresses automatically.  It uplinks to an 8-port hub which uplinks to a 4-port wireless router which has DHCP disabled.  The various computers are attached to the hub or the wireless router by cable.  We are not currently using any wireless connections.  All of our computers are configured to obtain an IP address automatically.

We also have a web server in the office, which was forwarded any http calls that were sent to our permanently assigned external IP address.  

Recently, we physically rearranged the room that contains the networking equipment.  In the process we powered down the network and unplugged many of the cables.  When we plugged them back in they may have gone to a different port or to the hub when they had been plugged into the router or vice versa.  (We can't think of any reason that the specific port would matter.)  Other than that, we did not (hopefully) change anything about the network.  

Since the move we have been having two problems:

1.  We are unable to get http forwarding to work.  Somehow it was apparently working in the past in conjunction with the dynamically assigned address.  After the move, the server received a different address but when we tried assigning http forwarding to that address it did not resolve the problem.

2.  We are getting error messages about IP address conflicts.  These are occurring on the server and one other machine.  How can this happen when all addresses are assigned dynamically?
Question by:monacoassociates
    LVL 7

    Expert Comment

    the 4 port wireless router that you said has dhcp disabled.  well, by ddefault those devices often have dhcp enabled.  take that router off the network and that should fix your problem.

    If it does,  then just turn it on, and hook one pc up to it, use the web int to reconfigure it.

    LVL 2

    Expert Comment

    Your server should have a static IP.  Also you'd get better network performance if you replaced the hub with a switch, even a cheap ($40) 8-port D-Link.
    LVL 2

    Expert Comment

    it seems you have a wiring problem than a logical problem please check the wiring from the DSL router to the hub abd from the hub to the wireless router.  

    it seems your PC's are actually not getting IP from the DHCP server they are getting automatic IP 169.254.xx.xx

    please check IP address of client computers if they start with the above it means they are not able to contact a dhcp server.

    check the cables for uplinks properly and then do something else.

    Please let me know so that i can advise further

    Author Comment

    Thanks to all for your input.

    tonyteri, our wireless router did have DHCP enabled, but we had already used the web interface to disable DHCP on it.  Shouldn't that keep it from assigning addresses or is there more than that one step involved?

    oceanjim, we have a 16-port D-link switch but haven't used it as yet.  It has no uplink, no designated ports--can you just plug anything into it anywhere?

    mivbinfotech, our computers are getting the IP addresses that the DSL router is programmed to assign (192.168.xx.xx).  The problem is one address is assigned twice.  As to your suggestion that we might have a wiring problem, we have had sporadic problems with connections, which we cannot isolate to one specific port or cable.  Could a cable have a loose connection so it works OK just part of the time?  Could the hub connections be temporarily disrupted somehow?  Sometimes certain computers will lose their connection in mid-session but it can then be re-established by a reboot (it doesn't seem to work to remap the drive to the network server).  

    LVL 2

    Accepted Solution

    This may be a little lenghty.

    The way I see it you should have the following setup Internet - DSL Router (DHCP disabled) - router (DHCP enabled) - switch - clients and web server.
    Another way to setup your network is assign everyone a static ip. As long as you don't expect your company to grow much You can eaily manage 18+ IP's. The only rule is to avoid IP's ending with: 0, 15, 16, 31, 32, 63, 64, 127, 128, 255. For example don't use

    To diagnose your IP issues (assuming this is a Windows enviroment) get to the command prompt (start - run - cmd OK).
    Type ipconfig /all.
    This will show you all you need to solved your problem. I'd look at your assigned ip address, if DHCP is enabled, if so what DHCP server assigned the IP, the date/time it was obtained, and the date it will expire.
    If you get a conflict on a machine you can type ipconfig /release this will drop your IP lease from the DHCP server.
    Then type ipconfig /renew this will request a new IP lease from the DHCP server in the segment.

    Once you can figure out who the DHCP server(s) are then you can attack the issue. You shouldn't have 2 DHCP servers in the same segment you will get conflicts.

    As to your question about the switch. Just plug it in and attach cilents, the switch will figure out who is who in 30 seconds or less. Basicly a hub repeats the same signal out every port no matter what the destination. A switch will figure out what is connected to it. On a switch if you try to send info to a known client only the ports involved will pass the traffic. If a switch doesn't know the destination it will act as a hub and send the packets to eveyone.

    Static IP's are simple for small companies. If you assign everone a static IP and you leave the DHCP servers enabled no computers will be asking for IP's and everything will work. If your company starts to grow you'll need a real server and then you can make it the DHCP server.

    If you're having connectivity problems (bad cables, connections) you can use the link lights on the switch to see if you are plugged in (orange is bad, green is good).

    Welcome to networking.
    LVL 2

    Expert Comment

    the problem is pretty clear the uplink port is not working properly it seems otherwise you have to physically check the computers with both ip addresses shut down all the PC's and then restart them one by one.  Also check whether it gives ip address conflict on either of the machines with the same ips.  
    There might be a possibility that the lease of the IP has expired on either computer that is the reason 2 computers got the same ip . you can renew the lease by typeing

    ipconfig /renew

    But the best thing is shutdown and restart again
    check the uplink port use cross over from any port of a switch or hub to any port on the switch or hub.

    Uplink should work properly.

    there is less possibility of cable loosening however it could be the problem so check the cable and put it tight on the Network card and switch or hub port.

    restart the whole setup including the routers switch and all the PC's server

    you can also try setting up static ip's on two PC's on different hubs/switches and ping each other to make sure your uplink os working properly between the switches/hubs

    This should solve the problem.

    Let me know so that i can advise you further

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