The network operating system makes a basis of any computer network. Each computer in a network is substantially independent, therefore the network operational system in a broad sense is understood as set of operational systems of the separate computers cooperating with the purpose of an exchange by messages and division of resources by uniform rules - to protocols. In narrow sense network OS is the operational system of a separate server providing an opportunity of a network functioning.

In network operating system of a separate server it is possible to allocate some parts:

• Control facilities local resources of a computer: functions of operative memory distribution between processes, planning and scheduling of processes, management of processors in multiprocessor machines, managements of peripheral devices and other functions of local OS resource management;
• Means of own resources granting and services in the general using - a server part of OS. These means provide, for example, blocking of files and records that is necessary for their sharing; conducting directories of network resources names; processing of the removed access inquiries to own file system and a database; management of turns of the removed users inquiries to the peripheral devices;
• Means of access inquiry to the removed resources and services and their uses - a client part of OS. This part carries out recognition and redirect in a network of inquiries to the removed resources from applications and users, thus the inquiry acts from the application in the local form, and is transferred in a network in other form corresponding requirements of a server. The client part also carries out reception of answers from servers and their transformation to a local format so for the application performance of the local and removed inquiries is indiscernible;
• Communication means of OS by means of which there is an exchange of messages in a network. This part provides addressing and buffering of messages, a choice of a route of the message transfer on networks, reliability of transfer, that is is means of messages transportation.

Depending on the functions assigned to a concrete computer, in its operational system can be absent either client, or server parts.

In practice there were some approaches to construction of network operating systems. The first network operational systems represented set of existing local OS and the network environment built on above it. Thus the minimum of the network functions necessary for work of a network environment which carried out the basic network functions was built in local OS. The principle of network operating systems construction in the form of a network environment above local operating system is used and in rather modern network operating systems. For example such as LANtastic or Personal Ware.

However the way of the operational systems development initially intended for work in a network is represented to more effective. Network functions at OS of such type are built deeply in the basic modules of system that provides their logic harmony, simplicity of operation and updating, and also high efficiency.

How functions between computers of a network are distributed, network operating systems and consequently, and networks share on two classes: one-rang and two-rang OS name two networks with the allocated servers is more often.

If the computer gives the resources to other users of a network it plays a role of a server. Thus the computer addressing to resources of other PC, is the client. As it has already been told, the computer working in a network, can carry out functions or the client, or a server, or to combine both these functions.

If performance of any server functions is the basic purpose of a computer such computer refers to as the allocated server. Depending on what resource of a server is divided, it refers to a file-server, a print-server or a server of applications.

On the allocated servers it is desirable to establish OS specially optimized for performance of server functions. Therefore in networks with the allocated servers network operating systems into which structure enters several variants of OS, server parts differing by opportunities more often are used.

The allocated server cannot be used as a computer for performance of the current problems which have been not connected with its basic purpose as it can reduce productivity of its work as a server. In connection with such reasons in Novell NetWare on a server part the opportunity of usual applied programs performance is not stipulated absolutely not. However in other network OS functioning on the allocated server of a client part is quite possible. For example, under control of Windows NT Server usual programs of the local user which can demand performance of OS client functions at occurrence of inquiries to resources of a network other computers can be started.

In one-rang networks all computers are equal access rights to resources each other. Each user can declare at the desire any resource of the computer divided then other users can maintain it. In such networks on all computers same OS which gives to all computers in a network potentially equal opportunities is established.

One-rang networks are easier in the organization and operation, however they are applied basically to association of small groups of the users who are not showing greater requirements to volumes of the saved information, its security from not authorized access and to speed of access. At increased requirements to these characteristics more suitable are two-rang networks where the server solves a problem of users service the resources as its equipment and network operational system are specially designed for this purpose is better.