I've had this question long ago and still I've been unable to find a correct answer to it. I know it's possible to run several virtual machines. Let's say that I'm running Windows Server 2008 R2 with two virtual machines running Ubuntu and Centos. How do IT manages to make the three operating systems (one physical, two virtuals) available to be accesed over the internet. Do they use the physical PC like a router that redirects traffic accordingly? Like ports 1-32766 to Machine 1 and 2-40000 machine 2? Thanks!
More often than not (highly dependent upon the specific requirements for the servers, of course) IT techs will establish the virtual servers with network interfaces (virtual or otherwise) that allow each server to have its own IP address. In this manner there is no need to divy up the ports as each system/virtual system has access to its own full range of ports.
In many of these configurations the virtual-machine software on the host machine acts as a network switch of sorts, directing traffic to the system/virtual system with the proper MAC address.
Some configurations utilize one or more virtual networks that exist wholly on the host machine in isolation from the real network. In this scenario the virtual-machine software on the host may act more akin to a router and direct traffic to the virtual networks/virtual IPs, or a virtual system itself may be configured with two interfaces and serve as a bridge between the real and virtual networks (often this system contains a DNS server).
Long story short, there are many different ways to implement multiple virtual servers on a single host!
For more detail, you may wish to read up on virtual network interfaces specific to the virtual-machine software in question, for example those of Oracle Virtualbox.