What is defined as a network security device that monitors and controls network traffic?

A firewall is specifically designed to monitor and control network traffic based on predetermined security rules. It acts as a barrier between a trusted internal network and untrusted external networks, filtering incoming and outgoing traffic to protect sensitive data and maintain the integrity of the network’s resources. Firewalls can be hardware-based, software-based, or a combination of both, and they enforce policies to decide whether to allow or block specific traffic.

While routers direct data packets between different networks, they do not inherently filter or monitor traffic for security purposes like a firewall does. Intrusion Detection Systems (IDS) also monitor network traffic but primarily focus on identifying potential security breaches, rather than controlling or filtering traffic directly. Switches operate at the data link layer and primarily manage the flow of data within a local area network (LAN) without focusing on security-driven traffic management. Thus, the role of a firewall in actively securing and controlling network access makes it the correct answer.