IP conflict in my network

hi, some users have IP conflict. Ip address are destrubuited dynamicaly by DHCP. why these users have this problem in my netwrok. do u think a virous cause this problem. how can I prevent it. thank for help.

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if a users changes his address manually to one, that has been given to someone else by dhcp, this will also happen. make sure they don't have admin rights.

or you are running out of addresses, check your dhcp server settings for free asddresses and leases.

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No virus has caused this problem. Either the DHCP server is not working right wherever it's at on the network or some machines are using static IP(s) in the range of the DHCP IP(s) that can be issued.

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Mr. Arnold6

Virus? Why does everyone think everything is caused by a virus? Most computer problems are caused by Microsoft, not viruses. Oh well.

A problem like this can be caused by the IP address lease running out, as far as the DHCP server is concerned, while the system that owns the lease is turned off. The DHCP server then hands out that IP to another system. The first system is then turned on, and bing, bang, boom you have an IP address conflict.

Check the DHCP server and extend the length of the IP address lease lifetime. Chances are it is too short for your needs. Then do an "ipconfig /release" on all systems, followed by an "ipconfig /renew".

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I dont know if i fully agree with the lease running out while the machines are turned off

If a lease was to run out while a pc was turned off and then consequently this lease was given to another computer, the original computer when it is turned on should be requesting a new lease as it knows that its lease has run out

Below is a url to dhcp guide

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It is still obviously a good idea to work out a good lease time that suits your needs but the dhcp server should handle the machines leases running out while turned off without problems

I would be looking to see that you dont have any pcs or printers or any other devices with static ip addresses and, if you are still having problems i would look at your pool of addresses and see if it is big enough for all your pcs and devices

Also I would check to see that you d> >

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That isn't what I have found. But I am thinking of an instance where a lightning strike took out 75% of the network cards at one company. All computers were off for several days, and when they were turned back on there were all sorts of IP address conflicts between the computers that still had their original cards.

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No, that should not cause any problems: if a device with an expired lease is switched on, it does a DHCPREQUEST for its old IP, but the DHCP server rejects if the IP has already been granted to another device. If the IP address has not been given to another device, it'll deliver a DHCPACK.

I've seen several causes for this problem:

  1. a PC with 'internet sharing' switched on will act as a DHCP server
  2. I've seen (very long ago) a switch (Cisco IIRC) to assign IP addresses to its own ports when given by a DHCP server
  3. The pool is too small
  4. like already suggested: someone is fiddling with fixed IP addresses


Reply to
Peter Boosten

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There can be weird issues with Windows and DHCP.

I work from home, and use a software VPN to connect to work. At night, I turn off the computer and the router (Linksys BEFSX41, the VPN on it is no longer used) and the modem (Motorola 5120). The router is set up with standard DHCP, using 192.168.3.xxx. The usual .1.xxx and .2.xxx addresses are set up for my backup DSL connection which has it's own modem and router (an identical Linksys BEFX41, but configured differently due to issues trying to communicate with the Siemens 4100 DSL modem). I don't use both simultaneously, but wanted to keep the routers separate.

For weeks, I had issues where the computer (running Win2K) would sit at 'Preparing Network connections' for as long as 2-3 minutes. I originally though this was due to it being part of a corporate domain and trying to look for something.

Also, every once in awhile, my VPN would drop and pop up the VPN login. A clue came when I looked at the VPN log, and saw a note that it had dropped because the IP address had changed to I also realized that this once happened exactly 24 hours after having turned on the computer at an odd time one day. The router is set to a standard 24 hour lease.

I changed the computer to use a fixed IP (outside the DHCP range), and all my problems disappeared. It boots up fast and hasn't dropped the VPN.

Reply to
Andrew Rossmann

I agree fully with what you have said, just one more point i would like to add,

I have seen this problem on a few occasions in a company that I previously worked for.

It happened at 2 different sites, we were getting ip conflicts with just 2 machines.

One of the problems was with a laptop and everytime it connected at a particular site it would give an ip conflict with another pc.

I traced the problem to this laptops network card having the same MAC address as one of the PC's network cards.

I know that every network card should have a unique MAC address but it happened a couple of different times at this company and each time it was the fact that the MAC address on 2 different NICs was the same.

I haven't seen this problem since though so I guess its fairly rare.

Peter Boosten wrote:

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Every network card should have a unique *default* MAC address. Probably the owner of the laptop changes his one to bypass a filter in the company's network.

Reply to
Sebastian Gottschalk

Are you sure that this conflict was not due to either party using software to spoof the MAC address?

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