Traditional packet filters are gradually becoming outdated, as even open-source solutions are shifting towards Next-Generation Firewalls. OPNsense stands out as a leading option for features like intrusion detection, application control, web filtering, and antivirus protection. No network, regardless of its size, is immune to potential attacks; even devices in home networks, such as washing machines and smartwatches, are at risk and need robust security measures. Firewalls play a crucial role in a comprehensive security strategy, shielding systems from both established and emerging threats. The effectiveness of a firewall is maximized when its capabilities are well understood, it operates intuitively, and is strategically placed within the network infrastructure. OPNsense rises to the occasion by fulfilling these essential requirements in various ways. This book serves as an invaluable guide for anyone looking to comprehend, install, and configure an OPNsense firewall effectively. Ultimately, understanding the intricacies of OPNsense can empower users to create a more secure digital environment.