During testing across the internet, which factor may cause inbound/outbound packets to be blocked and lead to inaccurate results?

• A. Firewalls
• B. Intrusion Detection Systems
• C. VPN
• D. NAT

Answer: (A)

Firewalls enforce access control by filtering packets as they pass through network borders. In internet-wide testing, a firewall can block inbound probes from reaching the target or block outbound responses from returning, causing probes to time out or appear as filtered. This direct blocking in both directions makes results misleading, since you might think a service is unavailable or misconfigured when it’s simply being blocked by the firewall.

Other factors like IDS, VPN, or NAT can influence traffic in different ways (monitoring, tunneling, or address translation), but they don’t inherently cause broad, bidirectional packet blocking in the way a firewall does.