Add the following directives with your desire IP inside the server section.
/etc/nginx/sites-available

server {
    allow 192.168.0.0; # here goes the IP you want to allow
    deny all;
    # ...
}

You can also write the directives inside the http section to apply it to multiple virtual hosts.

http {
    allow 192.168.0.0; # here goes the IP you want to allow
    deny all;
    
    server {
        server_name page1.rackspace.com;
        # ...
    }
    
    server {
        server_name page2.rackspace.com;
        # ...
    }
}