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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.

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Can any other computer connect via Wifi? – Mark Mar 28 '13 at 13:39

Under System Preferences > Network, choose AirPort, click on Advanced, select the name of your wireless network and click the - button to remove it. Turn AirPort off, wait a few, turn it back on, and try reconnecting to it.

You might also want to open the Terminal up and run the command:

dscacheutil -flushcache

Failing that, create a brand-new user account on the MacBook, log into that account, and see if it can connect.

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No good on all three counts: removing and re-adding the network didn't work, the Terminal command didn't work, and it can't connect on a new account. – daGUY Apr 6 '12 at 17:48
Have you tried the good ole SMC reset? After that I'd say boot from a known-good OS (ie, target disk mode off another Mac), or reinstall the OS. – da4 Apr 6 '12 at 18:22
Also FWIW the dscacheutil -flushcache command doesn't produce any output whatsoever. – da4 Apr 6 '12 at 18:22

Go to Preferences → Network → Wi-Fi → Advanced → DNS and remove any previous DNS servers that were listed. Apple then automatically sets default ones or uses the ones provided by your network.

I think the flush dscacheutil does not flush any old DNS name servers and inhibits Internet access.

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