What is a software layer

IT knowledge - what is a host computer

Host computer definition

A host computer (also host computer, host computer or host computer; loan translations from English host computer), or host for short, is a computer integrated into a computer network with an associated operating system that serves clients or houses servers (i.e. provides services).

As a guest at the host. What is a host?

In IT, a host computer (short: host) can basically be any resource that can accept another resource “as a guest” or supply the clients connected to it with certain IT services. The term first appeared in connection with mainframes: In this context, hosts are a central, intelligent hardware component of the mainframe that is available to all clients working on the system. When the PC hit the market, there was a smart device on every desk. The central mainframe intelligence was therefore no longer absolutely necessary for handling programs; the concept of master-slave / host-terminal has been replaced by client-server.


As a rule, the PCs in companies have been and are networked. In the client-server infrastructures that followed, there was also a central computer that provided services such as printing or communication via the network and was therefore actually the host of such applications. In contrast to the world of mainframes, this central computer was no longer called a host, but rather a server. After all, they wanted to demonstrate with regard to the naming that they were different and, of course, much more progressive. In contrast to the technology, little has changed here.


The host computer in the age of virtualization.

Then came the age of virtualization, and with it the term host was finally separated from its inseparable connection with a specific device on which the programs then ran. Which this computer hosted as a host. This is because virtualization separated the computer or the hardware, which used to be the only conceivable host for application programs (and still is today in the case of dedicated servers), from the software. It placed a new, special software layer, the so-called hypervisor, between the operating system running on the host computer and the software layers above it.


The software above is also changing: In virtualized environments it no longer consists of applications, but of virtual machines (VMs). They behave, and that is the key feature of this idea, now more or less like a host or server: You can run all sorts of programs on them, which such a VM then naturally hosts like the hardware computer used to do. At the same time, it is still said that the computer, i.e. the hardware, the operating system and the hypervisor running on it, host the VMs. In principle, this principle can be continued as desired, since an operating system with hypervisor could also be installed on the virtual machines.

Nested structures.

It's like the nested Russian wooden dolls. As is well known, they get smaller and smaller the more you work inward: All virtual machines that are on a piece of hardware share this hardware. And the more virtual machines run on a computer, which is its host, the fewer resources are available to the respective virtual machine. Although you can actually assign memory to a virtual machine, the memory bus does not become wider as a result. It is therefore an art to place the right number of VMs on a host, to dimension the host appropriately (i.e. to select a suitably powerful computer) and to pack the right applications into each VM. Because if the mix is ​​unfortunate because the applications in the VMs constantly access the same resources with the same (high) intensity, for example the memory buses, they slow each other down.


Dedicated, but without hardware.

The same applies, of course, to every virtual machine: In principle, several applications can be installed there too - although this is rarely done. The point of the virtual machine as a host for applications is precisely to have its own environment for an application, but not to have to invest in complete hardware that is often poorly exploited in non-virtualized environments. Also important: You can not only install the operating system running on the host hardware on the virtual machines, but everything that can somehow work with the hardware.


VMs can therefore be hosted on a host computer, on which, for example, "Microsoft Windows" and some Unix derivatives have been installed as the operating system. Virtual machines as hosts - a matter of course today Setting up virtual machines and thus using them as hosts for applications allow more or less all computer hardware and operating systems widely used in the professional sector. The technological pioneer and market leader in virtualized infrastructures based on hypervisors was and is VMware, although products such as “Microsoft Hyper-V” and the open source technology “KVM” have followed suit. In the meantime, the market has already completed the next expansion of the term host: In the cloud business, hosting is a business model.