Please triple-click anywhere in the line below on this page to select it:
/Library/Preferences/SystemConfiguration
Right-click or control-click the line and select
Services ▹ Open
from the contextual menu.* A folder named "SystemConfiguration" should open. Move the contents of that folder to the Trash, except the file with this name:
com.apple.Boot.plist
Leave that file where it is.
You may be prompted for your administrator login password. Restart the computer and test.
If there's no change, restore each of the items you deleted from a backup, overwriting the one that may have been created in its place.
If the problem is resolved, you may need to recreate some of your settings in the Network and Energy Saver preference panes.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You may not see what you pasted because a line break is included. Press return.