Unassigned IP Address

I recently upgraded to a gigabit router, with the intention of having my various computers and backup devices all wired in together and backing up at gigabit speeds. The router (a Billion 7800N) worked great, and the wireless-N speeds are very nice indeed. However, when it came to connecting to my Mac using an Ethernet cable all I got was:

Unassigned IP Address 169.x.x.x

I’d actually experienced this before while staying in a hotel and trying to use their free internet but had assumed it was the hotel’s problem. Not what I wanted to see from a new router.

Suggestions from the internet ranged from upgrading the firmware, rebooting the router, trying a new cable, and fiddling with various firewall options. Nothing worked. The strange thing was that running Windows via VMware Fusion on the same machine worked fine – Windows detected the connection instantly. It had to be a software issue.

After a bit of poking about I came across some Apple plist files that store network settings. A quick delete and reboot, and the connection was made without fuss. I can’t say why it wasn’t working, but somewhere along the line I’ve fiddled with something (not surprising, given the amount of software I install and remove), and it needed to be reset.

For future reference, and for anyone else who has the same problem, I deleted these files:

/Library/Preferences/SystemConfiguration/preferences.plist
/Library/Preferences/SystemConfiguration/NetworkInterfaces.plist

Remember that you will need to reboot afterward. OS X seems quite happy rebuilding the files, but if you want to play it safe you can copy them elsewhere before deleting.