Which describes a firewall?

• A. Intrusion Detection System
• B. VPN Gateway
• C. Firewall
• D. Antivirus Engine

Answer: (C)

A firewall is a network boundary device that controls traffic between networks by enforcing rules. It can be hardware or software and sits at the edge where two networks meet, such as between the Internet and the internal network. By comparing each packet or session against a defined policy, it decides whether to allow or block traffic, effectively filtering access to systems and data. In PCI DSS terms, firewalls are used to separate the cardholder data environment from other networks and to restrict who and what can reach those systems.

Intrusion Detection System detects and alerts on suspicious activity but doesn’t block traffic by default; it’s about monitoring and alerting rather than enforcing boundary control. A VPN gateway focuses on establishing secure, encrypted tunnels for remote connections, not on enforcing general traffic filtering between networks. An antivirus engine protects endpoints from malware, which is about file and program protection on devices, not about controlling network traffic at the network boundary.