If you would like to restrict access to a specific directory on your linux website you can create an .htaccess file within the directory with the following text:
Order Deny,Allow
Deny from all
Allow from 100.100.100.100
You will need to replace "100.100.100.100" with the ip you would like to have access to the directory. If your IP is dynamic, but within a specified range you can set it to something like 100.100. If you would like to add additional IPs you can do that with another Allow from line.