Lansweeper wasn't specifically designed for this. This feature is on our customer wish list, but we do not have a release date for it at this time. If you tick Save Pinged IP for your IP ranges under Configuration\Scanning Methods, Lansweeper will return a list of all pingable IPs. Returning a list of available, free IP addresses is theoretically possible if you add your own table to the database for instance that stores all possible IP addresses within the subnet. For now, there's no built-in functionality however to retrieve this information.