IP Conflict on Static IP

Mikejett
Mikejett used Ask the Experts™
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I have a server that has an IP address that was static but somehow changed after reboot was done on it.  The address was reserved on another DHCP server to prevent it from being used.  I am finding when I change the IP back to the static address it had, I get an IP conflict error showing up. I pinged the IP address on the LAN and I confirmed it is not being used by another device.  I was thinking DHCP is probably preventing me from using that address at this point so I tried disabling DHCP temporarily so I could then change that server IP back to it's static but I am still getting the conflict error.  

Any ideas what is causing this?
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Shaun VermaakTechnical Specialist
Awarded 2017
Distinguished Expert 2018

Commented:
Probably still a device on that IP. Test with something other than an ICMP ping.
Sr.Net.Eng
Top Expert 2011
Commented:
Check the DHCP server bindings on the dhcp server to see if it's holding it for another host.
If you have access to the layer 3 device routing for that network. Check it's ARP table to see if the ip address is assigned to a different mac address.
JohnBusiness Consultant (Owner)
Most Valuable Expert 2012
Expert of the Year 2018

Commented:
Also and as happened to me, you may need to delete your reservations, restart the server and make new reservations
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Commented:
If the duplicate IP device has a firewall, you can't ping it. You really have to resolve this another way. Even up to physically removing cables from the switch to trace this problem.

Author

Commented:
I found a MAC address assigned to that the IP after running a ARP.  Address is not that of the server.  Don't know how to detach it from the MAC.
atlas_shudderedSr. Network Engineer

Commented:
Trace it down through the MAC tables of the network.  Find the switchport it is connected to.  Clear the reservation from DHCP, shutdown the switchport.  Give it 10 seconds during which time you bring your server IP back up and online. Re-enable switchport and let the attached host grab a new address.
Prabhin MPDevOps Engineer
Distinguished Expert 2018

Commented:
use IP scanner in your network to check the IP address is occupied by any other user or not.

here is the List of IP scanner you can use,
https://www.advanced-ip-scanner.com/
https://angryip.org/download/#linux
Distinguished Expert 2018

Commented:
Someone else may have assigned that IP to another device. If you have managed switches, you should be able to track it down without too much trouble by port.
Top Expert 2007

Commented:
Do you have a DNS server with reverse lookup?

If the IP address is allocated by DHCP and being added to DNS then you may find the system hostname / computer name using reverse lookup (nslookup)

Author

Commented:
Thanks everyone.  I found it in the arp table and removed it.   that did the trick.
Distinguished Expert 2018

Commented:
Now the question is ask yourself is HOW it ended up in the ARP table
SouljaSr.Net.Eng
Top Expert 2011

Commented:
 "I found it in the arp table and removed it."

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Do you mean you used the arp table to track down the culprit device or just cleared it from the ARP table? Tracking down the device. Good. Just clearing it from the ARP table. Not good.

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