I can't access Facebook and Yahoo Mail on my MacBook Air.

Please read this whole message before doing anything.

This procedure is a diagnostic test. It’s unlikely to solve your problem. Don’t be disappointed when you find that nothing has changed after you complete it.

The purpose of the test is to determine whether the problem is caused by third-party software that loads automatically at startup or login, by a peripheral device, by a font conflict, or by corruption of the file system or of certain system caches.

Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards, if applicable. Start up in safe mode and log in to the account with the problem.

Note: If FileVault is enabled in OS X 10.9 or earlier, or if a firmware password is set, or if the startup volume is a software RAID, you can’t do this. Ask for further instructions.

Safe mode is much slower to start up and run than normal, with limited graphics performance, and some things won’t work at all, including sound output and Wi-Fi on certain models. The next normal startup may also be somewhat slow.

The login screen appears even if you usually login automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.

Test while in safe mode. Same problem?

After testing, restart as usual (not in safe mode) and verify that you still have the problem. Post the results of the test.

Restart your router and also your broadband device, if they're separate. I know your other devices aren't affected. Please do it anyway. If there's no change, see below.

Back up all data.

Unlock the Network preference pane, if necessary, by clicking the lock icon in the lower left corner and entering your password. Cllck Advanced, open the DNS tab, and change the server addresses to the following:

8.8.8.8

8.8.4.4

That's Google DNS. Click OK, then Apply.

In Safari, select

Safari ▹ Preferences... ▹ Privacy ▹ Remove All Website Data

and confirm. If you’re using another browser, empty the cache. Test. Any difference?

Notes:

1. If you lose Internet access after making the above change to your network settings, delete the Google servers in the Network preference pane, then select the TCP/IP tab and click Renew DHCP Lease. That should restore the original DNS settings; otherwise restore them yourself. Remember that you must click Apply in order for any changes to take effect.

2. I don't use Google DNS myself, though I have tested it, and I'm not recommending it or any other DNS provider; the server addresses are offered merely for testing purposes. There may be privacy and technical issues involved in using that service, which you should investigate personally before you decide whether to keep the settings. Other public DNS services exist.

Did I have to do that in Safe Mode?

No.

Double-click anywhere in the line of text below on this page to select it, then copy it to the Clipboard by pressing the key combination command-C:

www.facebook.com

Launch the Network Utility application.

Step 1

Select the Lookup tab and paste into the address field by pressing command-V. Then press return. Post the output that appears below – the text, please, not a screenshot.

Step 2

Select the Ping tab and do the same. Please enter the same input as you did in Step 1. Don't use the output of Step 1 as input to Step 2.

Important Note

Some web browsers and mail clients will automatically convert a domain name such as "www.example.com" to a clickable URL, such as http://www.example.com. That will interfere with the test. If necessary, edit the input in the Network Utility window to remove any added characters.

Is that what you get from Google DNS? What are your other devices?