How to enable NAT behind the cpanel ?
Most of webhosting providers currently used NAT service for their users. When you install cPanel & WHM, the installer will detect whether your server is on a NAT-configured network. If the installer detects a NAT-configured network, your server will configure itself for NAT mode and attempt to automatically map local IP addresses to public IP addresses.
The /var/cpanel/cpnat file acts as NAT mode. If the installer mistakenly detects a NAT-configured network, you can delete the /var/cpanel/cpnat file to disable NAT mode. You can rebuild cpnat if you need to enable it /scripts/build_cpnat script.
cPanel will attempt to map all non-loopback IP addresses bound to any network interface on the server to a public IP address.
cPanel server will send an outgoing connection from each local IP address to the http://myip.cpanel.net/v1.0/ server.
The build_cpnat script uses http://myip.cpanel.net/v1.0/ to map local IP addresses to public IP addresses.
You can manage the following options for local and remote IP addresses in WHM:
Add a New IP Address
Show or Delete Current IP Address
If multiple local IP addresses match the same public IP address which is NOT recommended, only the first local IP address listed in /var/cpanel/cpnat will be mapped.
If you wish to use a different IP address lookup service, open the /etc/cpsources.conf file with your preferred editor and add the following line:
Refer with cpanel documentation http://docs.cpanel.net/twiki/bin/view/AllDocumentation/NATSupport#Manage IP Addresses and Check CPNAT man page for further information.