We have a MAC G-5 in the office running Leopard OSX 10.5. Due
to a major storm, we shut all the machines down (common
practice here) until the storm passed. Upon bootup, at the
logon screen but above the logon dialog box, there appeared a
prompt which said:

MESSAGE:
IP 192.168.1.100 is being used by 00:15:99:26:60:ae

The machine is connected to a Linksys router via a RJ45 cable,
not via wireless. And 192.168.1.100 is the correct local IP.
It has never had a problem whatsoever before. Unless we click
on the "OK" button in that dialog box, we cannot
logon to the machine. And, after a successful logon, there is
no Internet connection, although the machine works fine
otherwise.

Anyone have an idea what this is about? We have tried
rebooting and doing a complete shutdown and startup but the
same message appears.

Jeff
[b][i]simul iustus et peccator[/i][/b]

I'd reboot the router. I saw this kind of shit on a Mac one time and a combination of rebooting machine and unplugging power from router and plugging it in again solved it.

It was an endless cycle of IP address conflict messages on the Mac, actually. Every one it got, it said was in use by "blahblah" on the network.

Perhaps something like this from the Windows world?

Third-party DHCP server may assign duplicate IP addresses to clients that are behind a network bridge
http://support.microsoft.com/kb/q303743/


Perhaps a router firmware issue?

You could also try to figure which particular device was 00:15 : 99:26:60:ae & tell it to stop using it ;-).