The book discusses the security of computer networks. Practical recommendations for conducting a penetration test of Cisco networks are given, useful information about the DTP protocol from Cisco Systems is given, a detailed guide to penetration testing of the network link layer, security testing and protection of MikroTik devices is presented. Describes the methods of conducting penetration tests with minimal damage to the network infrastructure. Tips on effective use of Kali Linux tools during penetration testing are given, the use of virtual machines for post-operation of systems is described. Describes recommendations for protecting network infrastructure from hacker attacks. Practical examples demonstrate pentest techniques for both attacking and defending sides. The electronic archive on the publisher's website contains color illustrations for the book.