I'm visiting my relatives and trying to get my MacBook on their network. If I connect over Ethernet, everything works fine and I can get online without a hitch. But if I connect over WiFi, I can't actually get on the Internet.
In System Preferences > Network, under AirPort, it says that it's using a "self-assigned IP address" and "will not be able to connect to the Internet." Furthermore, under Advanced > TCP/IP, the settings don't look correct when I compare them to the other computers in the house - the format of the IP address is different, the subnet mask is different, and the IP address of the router is blank.
I've tried copying the settings when connected over Ethernet (IP address, etc.) to use as manual settings for the WiFi, and it still doesn't work.
Other than this MacBook, there are two Windows laptops and two iPhones in the house (one theirs, one mine), and all of those other devices can get on the Internet over wifi with no issues. I'm stumped!
UPDATE: I never solved this, but the situation recently came up again – this time with a totally different computer – and I had the same exact issue: connects to the local network without a problem, but no Internet connection. So that tells me it's something wrong with the network/router, not the computer, but I still don't have any idea what it could be.