Your cPanel control panel is accessible via any of the following links:

http://yourdomain.com/cpanel
http://yourdomain.com:2082
http://yourip/cpanel
http://yourip:2082

Your WHM (WHM is only available for reseller, dedicated and VPS clients) control panel is accessible via any of the following links:

http://yourdomain.com/cpanel
http://yourdomain.com:2086
http://yourip/cpanel
http://yourip:2086


Your control panel/s run on ports 2082 (cPanel) and 2086 (WHM), so if you are behind a firewall or proxy server you might have trouble accessing your control panel if that port is blocked. Unfortunately, there is no way around this and the only resolution is to talk to your Network Adminstration about opening up that port for you.