Q My new broadband account went live some weeks ago, and whilst my Mac mini switched fine, my MacBook has been unable to connect to websites, collect email, or anything, although both connect to the D-link modem-router wirelessly. Even worse, my wife’s PC still running Windows XP also enjoys full Internet access. I have checked the MacBook’s network settings meticulously, and they appear correct, and essentially the same as the Mac mini’s. How can I get the MacBook online?

A There is one piece of ‘black magic’ that could resolve this nearly instantly. Remove the following preference files:

• com.apple.networkConfig.plist
• com.apple.airport.preferences.plist
• com.apple.network.identification.plist
• com.apple.PowerManagement.plist
• com.apple.NetworkInterfaces.plist

if you can find them in any of:

• /Library/Preferences
• ~/Library/Preferences
• ~/Library/Preferences/SystemConfiguration

and the following preference file:
~/Library/Preferences/SystemConfiguration/preferences.plist

Then restart with the Shift key held down (for Safe mode), and reconfigure the network connection. Restart in normal mode, and your MacBook may well work fine.

If that does not work, you will need to be more systematic, switching all your firewalls to block incoming connections only, not outgoing, and using fixed IP addresses rather than relying on DHCP. Select IP addresses on the same subnet as your modem-router, with masks of 255.255.255.0.

You can then check first that you can ping the MacBook from the Mac mini and vice versa, and next remote sites such as http://www.apple.com, using the Ping tab in Network Utility.

If this works for raw IP addresses but not longhand names, the problem lies in the Domain Name Server configured in the Network pane, which you should check against that advised by your ISP. Although you can revert from fixed IP addresses to using DHCP, the former has many advantages, in particular making it much easier to debug connection problems.

Fuller information on systematic diagnosis of network connection problems is here.

Updated from the original, which was first published in MacUser volume 27 issue 4, 2011.