Under normal situations, when you add a sub-domain or an addon domain, it uses the shared IP or the dedicated IP of the main domain. This post will cover on how to use a dedicated IP for an addon domain or even a sub-domain. This can be for a scenario when you require the sub-domain to have an SSL certificate.
# login to your server via SSH and edit the file /var/cpanel/useradata/user/addondomain.maindoman.com, replace the current IP shown with the dedicated IP which you intend to.
# Run /usr/local/cpanel/bin/build_apache_conf
# Login into WHM and navigate to Main >> DNS Functions >> Edit DNS Zone >> select the desired domain for editing and update the A records now to the new IP address and click save.
# In WHM go to Main >> IP Functions >> Show/Edit Reserved IPs
# Reserve the IP you just used so that no new accounts can use this IP address in the future.
# Restart httpd and DNS services.
This is all about assigning a sub-domain or an add-domain with a dedicated IP.