How do Servers handle the virtual machines?

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http://superuser.com – 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! (HowTos)