Simple packet filters are becoming a thing of the past. Even the open-source domain is moving towards Next-Generation Firewalls. OPNsense is a top player when it comes to intrusion detection, application control, web filtering, and Antivirus. No network is too insignificant to be spared by an attacker. Even home networks, washing machines, and smartwatches are under threat and require a secure environment. Firewalls are a component of the security concept. They protect against known and emerging threats to computers and networks. A firewall offers the highest level of protection if its functions are known, its operation is simple, and it is ideally positioned in the surrounding infrastructure. OPNsense accepts the challenge and meets these criteria in a number of ways. This book is the ideal companion for understanding, installing, and setting up an OPNsense firewall. Each chapter explains a real-world situation, describes the theoretical fundamentals, and presents a lab experiment for better understanding. Finally, it offers a solution using OPNsense methods and knowledge with a technical standpoint. The chapters are mostly independent of each other, however, they do increase in competency level. The topics covered are appropriate for beginners and professionals.