This can be a very effective way to protect your Joomla! administrator directory. Any other directory in public_html can be protected in the same way. This method only works if you have a static IP address assigned to you. Anyone attempting to browse such directories using a different IP Address will get a 403 Forbidden error.
Order Deny,Allow Deny from all Allow from 100.100.100.100
If comma separation does not work, then enter each IP address (or partial address) on its own "Allow from" line, such as:
Allow from 100.100.100.101 Allow from 100.100.100.102