This course is an introduction to network security, and primarily deals with the main techniques used in modern networks to overcome security problems, as well as with the scientific principles that are at the basis of such techniques. The first part of the course focuses on cryptography and cryptographic systems, which are seen as the main components of any modern approach to network security. The second part of the course introduces the main security mechanisms and cryptographic protocols applied to both IP networks and to telephone mobile networks. More than 25% of the course is devoted to laboratory exercises, where students will have a chance to configure and test many of the protocols and mechanisms that are explained in class.