Hello,
In order to assign the dedicated Ip to a sub-domain, please follow the steps link
1.Login to WHM
2.Follow the Tab :: Main >> DNS Functions >> Edit DNS Zone
3.select the main domain "mydomain.com" and add the A record for
mywebsite.com to the IP you want to assign
4.Then edit the DNS zone of "mywebsite.com" and point all the A record to dedicated IP.
5.Login to server with SSH and root user
6.Edit the file /var/cpanel/userdata/username/subdomain.domain.com file, change the IP and save it.
'username' represents the cPanel username of the account mydomain.com.
7. Run #/scripts/rebuildhttpdconf
8. Run #//etc/init.d/httpd restart
Hope this will help you