I'm having a really weird problem with my MacBookPro8,2. Ever since I updated to Mountain Lion a couple weeks ago I've been having this problem.
It will connect to my WPA network at home, I can hit my router's Web UI and browse around, but I can't hit anything outside the network. If I plug into the router I don't have such problems. None of my other computers have this problem (Windows 7, Chrome OS, and my wife's 4yr old MBP which was updated to Mountain Lion the same time as mine, none of them have this problem).
The weird thing is is that I don't have any problems on my office wifi. Only at home. I've tripple checked my Router's security settings to make sure I didn't accidentally block myself or something stupid: nothing like that.
Any ideas of what I can do to resolve this? Right now I just use either on of my worse laptops or just plug in the ethernet (lame).