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The basic idea behind virtualization is to use software to simulate the existence of hardware. This powerful idea enables you to run more than one independent computer system on a single physical computer system.

For example, suppose your organization requires a total of 12 servers to meet its needs. You could run each of these 12 servers on a separate computer, in which case you would have 12 computers in your server room. Or, you could use virtualization to run these 12 servers on just two computers. In effect, each of those computers would simulate six separate computer systems, each running one of your servers.

Each of the simulated computers is called a virtual machine or VM. For all intents and purposes, each virtual machine appears to be a complete, self-contained computer system with its own processor (or, more likely, processors), memory, disk drives, CD-ROM/DVD drives, keyboard, mouse, monitor, network interfaces, USB ports, and so on.

Like a real computer, each virtual machine requires an operating system to do productive work. In a typical network server environment, each virtual machine runs its own copy of Windows Server 2008 (or an earlier version). The operating system has no idea that it’s running on a virtual machine rather than on a real machine.

Here are a few terms you need to be familiar with if you expect to discuss virtualization intelligently:

  • Host: The actual physical computer on which one or more virtual machines run.

  • Bare Metal: Another term for the host computer that runs one or more virtual machines.

  • Guest: Another term for a virtual machine running on a host.

  • Guest Operating System: An operating system that runs within a virtual machine. By itself, a guest is just a machine; it requires an operating system to run. The guest operating system is what brings the guest to life.

    As far as licensing is concerned, Microsoft treats each virtual machine as a separate computer. Thus, if you run six guests on a single host and each guest runs Windows Server 2008, you need six licenses of Windows Server 2008.

  • Hypervisor: The virtualization operating system that creates and runs virtual machines.

    There are two basic types of hypervisors: Type 1 and Type 2. A Type 1 hypervisor is a hypervisor that itself runs directly on the bare metal. A Type 2 hypervisor is a hypervisor that runs within an operating system, which in turn runs on the bare metal.

    For production use, you should always use a Type 1 hypervisor because they’re much more efficient than Type 2 hypervisors. However, Type 1 hypervisors are considerably more expensive than Type 2 hypervisors. As a result, many people use inexpensive or free Type 2 hypervisors to experiment with virtualization before making a commitment to purchase an expensive Type 1 hypervisor.

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