No, using your example, you would be excluding for example 123.123.123.0, 123.123.123.1, 123.123.123.2, etc. through 123.123.123.255 so 256 IP addresses in total. Or to put it another way all IP addresses starting with 123.123.123.
When you log into cPanel you will see your accounts directory path displayed. It should be /home/cpanel-login. If you placed the script in the public_html folder then the path will be /home/cpanel-login/public_html/script-name.php
If you are still having problem please open a ticket with the help desk and they will help you further.