Other uses of this port became apparent later. A number of applications involve isolating users of virtual machines from each other and from the host.
There are several reasons to want isolation. One is to protect the physical machine and its resources from a potentially hostile process. The process would be run in a virtual machine which is given enough resources to run. Those resources would not be valuable, so they could be easily replaced if destroyed. It would be given a copy of an existing filesystem. If it trashes that filesystem, then it would just be deleted, and a new copy made for the next sandbox. It would have no access to valuable information, and its use of the machine's resources would be easily limited. The virtual Ethernet driver also makes it easy to control its access to the net. The daemon that does packet routing could be made to do packet filtering in order to control what traffic the sandboxed process is allowed to send and receive.
A variation on this theme is to put a non-hostile, but untrusted service in a virtual machine. A service is untrusted if it's considered to be vulnerable to being used to break into a machine. named is such a service, since it has had at least one hole which led to a spectacular number of breakins. An administrator not wanting to see this happen again would run named in a virtual machine and set that machine to be the network's name server. named requests from outside would be passed directly from the host to the virtual machine. So, anyone successfully breaking into that service would be breaking into a virtual machine. If they realized that, they'd need to find another exploit to break out of the virtual machine onto the host.
Another use of this isolation is to allocate machine resources, whether they be CPU time, memory, or disk space, between competing users. A virtual machine is given access to a certain amount of machine resources when it's booted, and it will not exceed those resources. So, if a user runs a very memory-intensive process inside a virtual machine, the virtual machine, and not the physical machine, will swap. Performance inside the virtual machine may be bad, but no one else using the physical machine will notice. The same is true of the other types of resources. CPU time can be allocated through the assignment of virtual processors to virtual machines. If a virtual machine is given one processor, it will never have more than one process running on the host, even if it's running a fork bomb. Again, life will be miserable inside the virtual machine, but no one outside will notice.
This level of isolation may find a large market in the hosting industry. Current hosting arrangements vary from application-specific hosting such as Apache virtual hosting to chroot environments to dedicated, colocated machines. Dedicated machines are used by customers who want complete control over their environments, but they have the disadvantage that they require a physical machine and they consume rack space and power. Running many virtual machines on a large server offers the advantages of a dedicated machine together with the administrative conveniences of having everything running on a single machine.