ServerBeat is a special protocol that will dynamically assing the specified IP between multiple CrushFTP servers.  You specify an IP that is free.  You then set any NAT port mapping to point to that IP.  The CrushFTP instances that are running configured for that virtual IP via serverbeat will share the IP.  Only one machine will have the IP at any time, and it will hold onto it unless its powered off.  Int hat case, within just a few seconds, the other CrushFTP servers will take over and start using the IP.

This is ideally suited for two machines in a failover scenario, but it could be used on 3 or 4 if you really wanted to.  Still only one machine is active at any time.  When Crush changes the IP, its changing it in the OS.  Technically its assigning an IP alias for the machine.