Implementing access control mechanisms like a firewall is a preventive measure in network defense. The preventive approach focuses on establishing barriers and safeguards to stop security incidents before they occur. Firewalls are a core component of this strategy, as they control incoming and outgoing network traffic based on predetermined security rules, thereby preventing unauthorized access to or from a network.

References: The Certified Network Defender (CND) program emphasizes a multi-layered defense strategy, which includes the Protect, Detect, Respond, and Predict framework. Within this framework, the Protect phase aligns with the preventive approach, as it involves the implementation of security measures that aim to prevent threats from compromising the network1. This is further supported by the NIST Cybersecurity Framework, which outlines Identify, Protect, Detect, Respond, and Recover as the five core functions for a comprehensive cybersecurity approach2.