Restrict access within website by IP address

Hi,
I’m looking for a way to restrict access to certain pages and posts/categories within a site, without forcing people to log in. I would like to filter by IP address. I’ve searched around, but haven’t found a solution. The Apache and .htaccess solutions all work per folder, which won’t work because the publicly accessible page/category/single.php files will be in the same folder as the restricted ones.
Does anyone know how I can accomplish this? An alternative would be to create a seperate subfolder or theme folder for the restricted users, but I don’t know how I could make that work.
Any help is greatly appreciated!!