Netdisco: Detect an IP Conflict

Overview

Detection/Symptoms

Most newer hosts (Windows, MACs, Linux) will actually detect a duplicate IP assignment and will generate a "Duplicate IP" error message. If a host gets a duplicate IP but its OS doesn't detect it, you'll see a valid address assignment, but network connectivity will just not work, or at best will be intermittent. You may also see intermittent "network unavailable" messages from the OS.

Mitigation

If you have access to a working host that is on the same VLAN as the duplicate IP, you should be able to detect the MAC addresses of the affected hosts by checking its ARP cache. The ARP cache of a machine contains IP to MAC address mappings, which every host needs to communicate with hosts on the same network as its own. Here is how you can check it:

1. On an unaffected host on the same network, open up a command prompt.
   • On a Windows machine, type "arp -a [suspected duplicate IP]" and hit enter.
   • On a Mac or Linux machine, type "arp [suspected duplicate IP]" and hit enter.
2. The output of the response should contain a MAC address. Compare this address to the affected machine. Is it the same one?
   • If not, then the MAC address in the output is the MAC of another machine that has the same address. You should be able to use NetDisco or DCT to track that host down via MAC address.
   • If so, then try running the command a few more times. If the MAC changes, then you know another machine is trying to use the address. You should be able to use NetDisco or DCT to track that host down.
   • If the MAC address in the output is always the affected host, try unplugging that host from the network. Wait a few minutes, then run the ARP command again. If you get any MAC address in your output, it should be from another host using that IP.

If you don't have access to a third host on that same network or don't have command line access, submit a help request ticket and assign to ITS - Network. Provide them with the suspected duplicate IP and they can investigate. For urgent issues during business hours call 734-647-4200.