The main tasks of system administration are outlined, basic protocols are described, and recommendations are given for choosing equipment and carrying out daily routine operations. The technologies used in the construction of information systems are described in detail, and the means for monitoring and maintaining both small and distributed networks are described. Methods of centralized management and the basics of creating a safe environment are considered. Recommendations are given for troubleshooting and ensuring data protection. In parallel, solutions based on the Windows and Linux operating systems using both proprietary and open technologies are considered. The book is written on the basis of many years of experience in the development and practical administration of information systems. In the sixth edition, the material is focused on modern versions of the Windows 10 and Windows Server 2016/2019 operating systems. Irrelevant material has been excluded, useful practical examples have been added on SSL support for the Squid proxy server, using Windows Sandbox, etc. Particular attention is paid to the choice of a DLP data leakage prevention system.