In order to restrict Joomla Administrator section to only one or few particular IP’s add the below in the .htaccess file in the administrator directory

Order Deny,Allow
Deny from all
Allow from <IP 1>
Allow from <IP 2>
Allow from <IP 3>
Allow from <IP 4>

Note:

The code should be added only in .htaccess file under administrator directory and nowhere else.

Replace <IP 1>, <IP 2>, <IP 3>, <IP 4> with your real IP’s.