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Windows Thread, Duplicate IP address on 2 machines DNS Issue? in Technical; Hi Seem to be having a problem where occasionally 2 computers may get the same IP address. Problems we begin ...
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    ranj's Avatar
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    Duplicate IP address on 2 machines DNS Issue?

    Hi

    Seem to be having a problem where occasionally 2 computers may get the same IP address. Problems we begin to see are things like when using VNC we intend to connect to a particular machine but instead get connected to another machine on the network.

    We can resolve it by running an ipconfig /release and an ipconfig /renew on both machines and it sorts it but then we can have the problem reappear on other machines on the network.

    I have checked how many IP addresses are leased out and we are barely using 50% of our allocation so I know that we aren't running out of IP addresses.

    Does anyone have any suggestions on what else we could try?

    Thanks

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    waldronm2000's Avatar
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    Is it always the same two computers or does it move around? How many DHCP servers do you have? What are you running your DHCP on?

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    Michael's Avatar
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    How many DHCP Servers do you have? If you have more than one you need to enable collision detection.

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    ranj's Avatar
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    We are running it on a Windows 2008 server. We only have 1 DHCP server on the domain.

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    waldronm2000's Avatar
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    With only one DHCP server you shouldn't get this problem. Any chance DHCP is enabled on another device, such as a router? One thing to check next time it crops up is IPCONFIG /ALL on both computers that are conflicting. Check for a MAC address conflict (long shot, but that's why I was asking if it's the same computers every time), and check the DHCP server address reported by both hosts.

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    glennda's Avatar
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    Do you have 2 DNS servers?

    And when using VNC are you entering the Ip address or computer name?

    If computer name then it sounds as if the DHCP server is not updating the DNS server/s - in the settings you can force DHCP to update DNS even when the client doesn't request it - i would check this as it sounds as if the DHCP server is giving out the addresses but the DNS server is not being updated with the new address of the machine.

    Also how long a lease do you give the machines?

    Toby

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    waldronm2000's Avatar
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    I just re-read your original post and spotted that it's a roaming problem, so not going to be down to MAC addresses. Check the DHCP server addresses reported by IPCONFIG/ALL, as mentioned above. Failing that, I would suspect DHCP database corruption.

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    We had this problem once before, specifically with VNC - it turned out we needed to enable scavenging on the DNS to ensure that old IP addresses were removed.

    Do you leave your PCs on permanently or are they shut down regularly?

    Peter

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    Quote Originally Posted by howartp View Post
    We had this problem once before, specifically with VNC - it turned out we needed to enable scavenging on the DNS to ensure that old IP addresses were removed.

    Do you leave your PCs on permanently or are they shut down regularly?

    Peter
    This would be what i mean - but if you have the DHCP Automatically updating the DNS entry then you will only have old Dns entries for machines that are removed/rebuilt.

    If you are not careful enable Savaging of the records will delete manual entries in there for staticly assigned servers that do not try to update there dns entries.

    Have you also tried it from mulitple Machines? as if its just your machine and it hasn't been restarted or something like that the the DNS cache on the local machine might be affecting it.
    Toby

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    waldronm2000's Avatar
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    Ah yes, just realised from other posts what you mean by duplicate IP addresses - actually duplicate host records in DNS, not computers with identical leases. Scavenging is the solution.

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    ranj's Avatar
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    Ahh yes I was under the impression that scavenging was set but I think that was when we had a 2003 network. I dont think I enabled it when we moved over to 2008 so have set that now. I'll see how we get on and report back. Thanks for the replies.

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    Quote Originally Posted by waldronm2000 View Post
    Ah yes, just realised from other posts what you mean by duplicate IP addresses - actually duplicate host records in DNS, not computers with identical leases. Scavenging is the solution.
    This sounds bang onto me. If you were connecting with a hostname rather than an IP, then if the records in DNS weren't being scavenged, then I would expect to see this.

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