At the file "/modules/servers/freenas/ipconfig.php" you can specificate which IPs should be allowed by default for

The IPs listed in this file will be allowed when a service get activated.

function freenas_setAllowedIPs ($pid) {
    return array ( 
				   'nfs' => array (''),
                   'cifs' => array(''),
                   'iscsi' => 'ALL',

The file is open for modifications, so you can integrate your own code to return other IPs based on the service id.