Problem:

How do I prevent unauthorized users or guests from accessing the administrator login screen?

Solution:

Create an .htaccess file in the administrator folder. Add the following to it, and nothing else:

Order Deny,Allow
Deny from all
Allow from 00.000.

Replace the red 0s with the first 2 parts of your IP address. For example:

Order Deny,Allow
Deny from all
Allow from 127.0.

You can add as many IP addresses as you want. Just create a new line for each. For example:

Order Deny,Allow
Deny from all
Allow from 127.0.
Allow from 192.168.

To remove access to an IP address, simply delete the line that needs to be removed, or put a hash (#) in front of the line. For example:

Order Deny,Allow
Deny from all
Allow from 127.0.
#Allow from 192.168.

When visitors try to access the administrator, they will either see a Forbidden error message or a 404: Page not found. If you receive the same errors, you need to verify your IP address is up to date in the .htaccess file. Internet service providers sometimes change the IP address of your Internet service.

This will work for any directory on any website, not just Joomla.