If you or your web hosting clients are behind strict firewalls and can only use port 80 for accessing the cPanel control panel, you can install a program called cpanelproxy to achieve full access

Installation instructions:
- Create subdomain "cpanel.yourdomain.com" (You'll probably need access to cPanel for this, so install it before you need it!)

- Download and unzip cPanelProxy.zip from http://cpanelproxy.net/

- Upload the files to the subdomains directory, probably "public_html/cpanel" in FTP. Be sure to remember .htaccess!
Needed files: cpanelproxy.php .htaccess php.ini

- You likely will want to edit the .htaccess file and remove any references to php4 since your server likely is php5 only.

Install for webmail and WHM:
- Repeat with "webmail" instead of "cpanel" in subdomain and directory.
- Repeat with "whm" instead of "cpanel" in subdomain and directory.

Now your users can go to cpanel.domainname.com webmail.domainname.com and whm.domainname.com and stay purely on port 80.