Virtual operating systems, also known as virtual machines, are becoming a core component of the IT infrastructure. It is the most prevalent form of virtualization in use. Virtual machines are typically full implementations of standard operating systems, such as Windows Vista or Red Hat Enterprise Linux, running simultaneously on the same physical hardware.

Application server virtualization is a synonym for advanced load balancing. It is a one-to-many virtualization representation: one server is presented as a virtual interface, hiding and balancing the availability of multiple web servers or applications as a single instance. This provides a more secure and efficient topology than allowing direct access to individual web servers.

Application virtualization is equivalent to the longstanding utilization of thin clients. The local workstation provides the CPU and RAM required to run the software; however, nothing is installed locally on the machine.

Browser-based applications are implementations of application virtualizations; the applications run locally on the workstation and the management and application logic execute remotely.

Management virtualization is an integral component in data center management and the segmentation of administration roles. Network administrator roles can be defined with full access to the infrastructure routers and switches, but without administrative-level access to servers.

Network virtualization is implemented in the form virtual lP management and segmentation. A VLAN is a single Ethernet port supporting multiple virtual connections from multiple IP addresses and networks which is virtually segmented using VLAN tags. Each virtual lP connection over this single physical port is independent and unaware of the existence of other connections; however, the switch is aware of each unique connection and manages each one independently.

Virtual routing tables also are a form of network virtualization. Virtual routing tables provide a one to many relationship, where any single physical interface can maintain multiple routing tables, each with multiple entries. This provides the interface with the ability to dynamically bring up and discard routing services for one network without interrupting other services and routing tables on that same interface.

Hardware virtualization subdivides components and locations of physical hardware into independent segments and manages those segments separately. Asymmetric multiprocessing is a form of pre-allocation virtualization where certain tasks are only run on certain CPUs. In contrast, symmetric multiprocessing is a form of dynamic allocation, where CPUs are interchangeable and used as needed by any part of the management system.

Each classification of hardware virtualization is unique and has value, depending on the implementation. Both symmetric and asymmetric multiprocessing are forms of hardware virtualization. The process requesting CPU time is not aware which processor its going to run on; there is a request for CPU time from the OS scheduler and the scheduler takes the responsibility for allocating processor time. From the perspective of the process, processor time could be spread across any number of CPUs and any part of RAM.

Pre-allocation virtualization is well suited for specific hardware tasks, such as offloading functions to a highly optimized, single-purpose chip. However, pre-allocation of commodity hardware can cause artificial resource shortages if the allocated chunk is underutilized.

Dynamic allocation virtualization is a more standard approach and typically offers greater benefit when compared to pre-allocation. For true virtual service provisioning, dynamic resource allocation is important because it allows complete hardware management and control for resources as needed; virtual resources can be allocated as long as hardware resources are still available. The downside to dynamic allocation implementations is that they typically do not provide full control, leading to processes which can consume all available resources.

There are two general classes of storage virtualization: block virtualization and file virtualization.

Block virtualization utilizes SAN: Storage Area Network and NAS: Network Attached Storage technologies and presents itself as a single physical device. SAN devices make use of another form of storage virtualization, RAID: Redundant Array of Independent Devices.

SCSI: Small Computer System Interface is a common implementation of block virtualization; it allows an operating system or application to map a virtual block device, such as a mounted drive, to a local hardware or software network adapter instead of a physical drive controller. The SCSI network adapter translates block calls from the application to network packets the SAN understands and then back again. It provides a virtual hard drive.

File virtualization moves the virtual layer up into the file and directory structure level. Most file virtualization technologies serve as an interface to storage networks and keep track of the files and directories which reside on storage devices and maintain global mappings of file locations. When a request is made to read a file, the user will work with the file as if it is statically located on their personal remote drive. However, the file virtualization appliance recognizes that the file is physically located on a server in a data center at a different geographic location. File-level virtualization renders the static virtual location pointer of a file immaterial to its physical location, allowing the back-end network to remain dynamic. If the IP address for the server is changed or the connection needs to be re-routed to another data center, only the virtual appliances location map needs to be updated.

Service virtualization consolidates operating system, application server, application, management, network, hardware, virtualization, and storage virtualizations. Service virtualization connects all of the components utilized in delivering an application over the network; it includes the process of making all pieces of an application work together regardless as to where those pieces physically reside. Service virtualization is typically used as an enabler for application availability.