The NAT service was provided by the natpmpd binary in Lion Server and the natd binary in Mac OS X Server v10.6. In Mountain Lion, natd is deprecated and natpmpd is used by Internet Sharing in the Sharing pane of System Preferences.
You can enable Internet Sharing to provide NAT services using the default IP address range of 192.168.2.2 to 192.168.2.254. Important: Do not enable Internet Sharing if you have configured custom settings for the DHCP service in OS X Server. Any existing DHCP settings will be overwritten if you enable Internet Sharing in the Sharing pane of System Preferences.
If you previously enabled NAT in Server Admin in Lion Server, your natpmpd configuration will not be preserved when you upgrade to OS X Server (Mountain Lion). You can restore the default NAT and DHCP configuration by disabling then enabling Internet Sharing in System Preferences.
If you enabled NAT in Mac OS X Server v10.6, your natd configuration will be deactivated when you upgrade or migrate to Mountain Lion. Your configuration files will be preserved in /Library/Server/Migrated/private/etc/nat/.
Custom NAT configurations can be configured using the pfctl command. For instructions, please see man pfctl.
If you created NAT rules with pfctl in Lion Server, they will be preserved and remain active after upgrade or migration.