# Mac OSX: "IP address is in use by" error



## sjmoll (Apr 16, 2003)

I do tech support for internet access in hotel rooms. Occasionally, we've seen a problem with Macintoshes where in getting an IP address from the DHCP server, they will get the following error:
"[IP Address] is in use by [MAC Address], DHCP server [Server's Address]"

When the user clicks ok on the error window, they get the same message back, but with a new IP address. For the current user I'm trying to help, it is always the same MAC address that is coming up (I don't know for sure if that is always the case).

This problem only happens with some Macs. Previously, we have assigned the user a static IP address, and they have been able to get on fine, but thats just a workaround. We'd like to find out what causes this problem, and if there is something else that can be done to correct the problem. The current user is trying to connect through his Airport card (though I'm pretty sure others were getting it through their Ethernet card).

We've tried all the standard things, like rebooting, setting a static IP and switching back to DHCP, etc. None of that has resulted in anything different. Any ideas would be greatly appreciated.

Thanks
Shawn


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## bluesangria (Aug 1, 2003)

Since you stated the problem only occurs with Macs, and only occassionally you may want to check the following:

Make a note of the Airport MAC address and the built-in Ethernet MAC address. Don't laugh, but I have seen it happen where, if you assign the same IP to both interfaces, they will conflict with each other. I believe the problem was fixed in later versions of OS X.

Workaround: Disable one or the other interface or assign a bogus static IP to one, and set the other to DHCP.


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## sjmoll (Apr 16, 2003)

Actually, we did check that. The MAC address he's getting in the message is not associated with any other adapters on his PC.

Thats a good thought though to try setting the other one to static and see what happens. Unfortunately, that would still be a workaround, but at least we'd have narrowed it down to an OS problem.


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## sjmoll (Apr 16, 2003)

Setting a static IP to his ethernet and leaving the airport on DHCP did not have any affect. Any other ideas?


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## bluesangria (Aug 1, 2003)

Well, it's not very fun, but when I have a persistent IP conflict and I don't have a list of MAC addresses, then I go back to the beginning. Ping the IP address that's supposed to be in use, then look at your ARP table and see if that actually matches the MAC address involved in the conflict. I usually telnet into my switches and look at their ARP caches to see if the MAC address originates from any of their ports. You can also try to cross-reference the vendor of the MAC address. This link is just a big text file of vendor listings and their assigned MAC addresses. It might give you another clue as to what machine its conflicting with.
Just had a thought. What happens if you manually assign the IP address that the Apple WOULD have gotten from the DHCP server? Does it still conflict? If you try to verify whether or not there is an actual IP conflict and there is not, perhaps it's some nuttiness between the Apple and the DHCP server. As an aside, I've heard that Windows machines tend to boot back with the DHCP address they had last, even if its not assigned to them. Don't have too much experience with that though.
Another thought, is it possible someone is statically using the same range of IP addresses assigned to your DHCP server?


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## sjmoll (Apr 16, 2003)

Well the guest has checked out now, but we get this problem every once in a while at the Hotels we support.

No one should have been using a static IP address there, though guests will sometimes do it on their own. In the past, assigning a static IP address has always fixed the problem. I dont think we've been assigning the address the DHCP was giving them.

Thanks for the rest of your suggestions as well. Let me know if you think of anything else.


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