Computer networks are the spinal cord of our civilisation; because of the huge stakes involved, universities and certain companies must now teach the basics of computer system security. The purpose of this book is to help people learn about the fundamentals of this field through the study of solved exercices. The primary goal is not to create experts in this domain, but rather to teach the concepts required to understand the risks and consequences of computer system misuse. This book thus handles the basic aspects of computer security, including e-mail; viruses and antivirus programs; program and network vulnerabilities; fire walls, address translation and filtering; cryptography; secure communications; secure applications; and security management.This work is a novel addition to the literature in that it gives, for each subject, a short summary of the main principles followed by a series of exercises (with solutions), resulting in a self-contained book that provides a solid foundation in computer security to students. Chapters in this book have been conceived to be as independent as possible. All of them, nevertheless, require a background in computer science and a familiarity with the most common network protocols.