Which security device monitors and filters traffic between networks according to a set of rules?

• A. Firewall
• B. Router
• C. Switch
• D. Load Balancer

Answer: (A)

A firewall is the security device that monitors and filters traffic between networks according to a set of rules. It sits at the network boundary and enforces security policies by inspecting each packet or flow and deciding whether to allow or block it based on criteria such as source and destination IP addresses, ports, and protocols. Stateful firewalls keep track of active connections to ensure return traffic is legitimate, while stateless ones apply rules to individual packets. This filtering behavior distinguishes it from a router (which mainly directs traffic), a switch (which forwards frames at the data link layer), and a load balancer (which distributes requests across servers). In practice, firewalls enforce what traffic is permitted between networks, providing the control point for security boundaries.