Which statement describes a firewall and an intrusion detection system?

• A. A firewall filters network traffic by rules.
• B. An IDS monitors for suspicious activity and raises alerts.
• C. A firewall blocks all traffic by default.
• D. A firewall filters network traffic by rules; an IDS monitors for suspicious activity and raises alerts.

Answer: (D)

Firewalls and intrusion detection systems address different parts of network security. A firewall serves as a gatekeeper, filtering traffic according to predefined rules—allowing some flows and blocking others based on criteria like source and destination addresses, ports, and protocols. An intrusion detection system watches for patterns of activity that look suspicious or abnormal, such as unusual login attempts or port scans, and raises alerts when it detects potential issues. The best description combines both roles: a firewall filters network traffic by rules, and an IDS monitors for suspicious activity and raises alerts. The other options miss one of these functions or make an inaccurate claim (for example, that a firewall blocks all traffic by default, which isn’t a universal rule and would disrupt normal communication).