Is that possible to make child portal url (parent-portal.com/abc) to be like 'abc.parent-portal.com' ? 

I know this could be achieve by simply creating subdomain through hosting provider cpanel and then creating the parent portal for that new subdomain. But it is not my option because it will require hosting provider or host administrator to create the subdomains first. I want my users can simply register new portal without the need for me to create subdomain for them first.

Any ideas on how to achieve this? can i use url rewriter or other 3rd party modules? Any references would be highly appreciated. Thanks