1

On my work network, when I go to my DHCP server, then open DHCP > domain_name > IPv4 > Address Leases, I found that 192.168.160.7 is assigned to a iPad device (connected via Wifi).

When I use command prompt and type "arp -a" I found that 192.168.160.7 is mapped to a MAC address which is a desktop computer.

How is this possible? How do I know which device really has that IP?

Thank you in advance!

3
  • 1
    this is likely due to the IP address statically set on the desktop computer. Check this first. – JFL Oct 4 '16 at 7:16
  • 2
    how exactly did you determine that one is an iPad and the other is a desktop? – hertitu Oct 4 '16 at 8:19
  • Did the answer help you? If so, you should accept it so that the question doesn't keep popping up forever, looking for an answer. If you have answered on your own, please post an answer and accept it. – Ron Maupin Aug 2 '17 at 13:40
1

It is possible your PC has an old ARP entry in the table. If the PC did not perform an ARP request for this IP address recently, while the IP was assigned to a new device.

If you ping the address, your ARP table should update with the new MAC address after the original ARP entry has expired due the aging timer.

This is of course, providing you do not have an IP conflict and the desktop has a static IP address assigned while the DHCP gave out the same IP address to another device.

7
  • Why and how would the PC update it's ARP table when you ping? – hertitu Oct 4 '16 at 11:20
  • 1
    When you PING the IP address on a LAN, the PC needs to know the MAC address of the device it is trying to PING. To find out, it will send an ARP request as a broadcast, asking for anyone with that IP address, to send their MAC address to your device, which then get's placed in the local ARP table. If the IP address of the remote device changes, the mapping between MAC and IP must also be updated. On a LAN which will make the PC do another ARP to get the updated MAC, you are actually communicating via MAC addresses because it is considered layer 2 but for human eyes we use IP to make it easier. – SleepyMan Oct 5 '16 at 12:54
  • You didn't answer my question, let me rephrase it. When the PC already has the (old) IP address-MAC address combo in its ARP table, what triggers it to send another ARP request? – hertitu Oct 5 '16 at 13:33
  • 1
    So If you ping the address, your ARP table should update with the new MAC address in your answer is incorrect. In the situation that you describe in the answer (when the ARP cache contains an entry) the ping will not trigger an ARP request. – hertitu Oct 5 '16 at 14:14
  • 1
    Perhaps you should edit your answer to correct that then? – hertitu Oct 6 '16 at 7:38
1

You aren't providing a lot of information to go on, but I will assume this is a relatively simple flat network.

How is this possible?

There are a few possible causes.

You may have an IP conflict on your network. One device (the iPad) is learning it's IP address from DHCP. The desktop is configured to use a static IP address instead of DCHP. Both are trying to use the same address.

It is also possible that the DHCP server/service was restarted and doesn't maintain state between restarts (i.e. many consumer devices). It may have provided the IP address to the desktop prior to the restart, then provided it to the iPad after the restart. The desktop will still think it has a right to the IP address for however long it was given the lease. This also results in an IP conflict, but it will resolve itself when the desktop goes to renew it's DHCP lease.

The other option is that the device you ran your "arp -a" command on has a outdated entry for the IP address referencing the desktop. Most devices only maintain dynamic ARP entries for a relatively short period of time (a few minutes), so you may have just happened on a normal situation in the relatively narrow window it could exist. Or potentially, you could have an outdated static ARP entry on the device you ran your "arp -a" command, but this is unlikely unless your environment normally makes use of static ARP entries.

How do I know which device really has that IP?

Likely both devices have that IP address (first two reasons above) which will cause problems. How to deal with it depends on the situation.

Easiest is if the DHCP service was restarted and both devices learned the IP from DHCP. This will either resolve itself given time, or you can force the desktop to request an IP address (disconnect and reconnect it to the network or use a method present in the OS such as "ipconfig /renew" from the Windows command prompt).

If the desktop is configured with a static IP address, you would need to determine if it should be configured statically or not. If yes, then the IP address should be removed from the DHCP pool of addresses so the DHCP server does not assign it to another device. If no, then the desktop should be configured to use DHCP to learn it's IP address.

Finally, if you have an old ARP entry, either wait a couple minutes (most dynamic ARP entries have fairly short aging periods) or a static ARP entry would need to be removed.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.