This textbook discusses core topics in computer network security, providing a single source that covers network security issues and uses the most common network protocols as detailed examples. It explores the International Standards Organization's Open Systems Interconnection (ISO OSI) network stack and discusses common security weaknesses, vulnerabilities, attack methods, and mitigation approaches. The risks and mitigation options for each layer of the ISO OSI model are covered in general terms, and specific protocols are provided as examples and case studies.