Quick Answer
The correct answer is a packet filter firewall. This type of firewall operates at Layer 3 (Network) and Layer 4 (Transport) of the OSI model, making decisions based solely on source and destination IP addresses, ports, and protocols—exactly as described in the scenario where traffic is allowed by source IP only, with no application-layer inspection. On the CISSP exam, this question tests your understanding of firewall types and their OSI model placement; a common trap is confusing a packet filter with a stateful firewall, which tracks connection state, or an application-layer gateway, which inspects payloads. To distinguish them, remember that a packet filter is stateless and blind to Layer 7 content. A useful memory tip: “Packet filters peek at the packet’s envelope (IP and port), not the letter inside.”
This CISSP practice question tests your understanding of communication and network security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
During a security audit, it is discovered that a network firewall is allowing traffic based on source IP address only, without inspecting application-layer data. Which type of firewall is this?
Packet filter
Packet filters examine only packet headers.
Circuit-level gateway
Why wrong: Circuit-level gateways validate TCP handshakes but do not inspect headers deeply.
Application gateway
Why wrong: Application gateways proxy and inspect application-layer data.
Stateful inspection
Why wrong: Stateful firewalls track connection state, not just header fields.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Packet filter
A packet filter firewall operates at Layer 3 (Network) and Layer 4 (Transport) of the OSI model, making decisions solely based on source and destination IP addresses, ports, and protocols. It does not inspect application-layer data (Layer 7), which matches the scenario where traffic is allowed based on source IP address only. This is the simplest and fastest type of firewall, but it lacks the ability to block attacks embedded in application payloads.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Packet filter
Why this is correct
Packet filters examine only packet headers.
Related concept
Read the scenario before looking for a memorised answer.
Circuit-level gateway
Why it's wrong here
Circuit-level gateways validate TCP handshakes but do not inspect headers deeply.
Application gateway
Why it's wrong here
Application gateways proxy and inspect application-layer data.
Stateful inspection
Why it's wrong here
Stateful firewalls track connection state, not just header fields.
Common exam traps
The trap here is that candidates often confuse 'stateful inspection' with 'packet filtering' because both examine IP addresses, but stateful inspection also tracks connection state, whereas the question explicitly states no application-layer inspection and only source IP filtering.
Detailed technical explanation
Packet filter firewalls use access control lists (ACLs) to match packet headers against rules, often implemented via iptables (Linux) or ip access-group (Cisco IOS). A subtle behavior is that they can be vulnerable to IP spoofing attacks because they do not verify whether the source IP is legitimate beyond the header. In real-world scenarios, a packet filter might allow traffic from a trusted source IP like a partner network, but an attacker could spoof that IP to bypass the firewall and deliver malicious payloads.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A security analyst at a medium-sized enterprise encounters this scenario during an investigation or architecture review. The correct answer reflects best practice for the specific threat or control described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Security exam questions test whether you can match controls to threats in context — not just recall definitions.
What to study next
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Communication and Network Security — This question tests Communication and Network Security — Read the scenario before looking for a memorised answer..
The correct answer is: Packet filter — A packet filter firewall operates at Layer 3 (Network) and Layer 4 (Transport) of the OSI model, making decisions solely based on source and destination IP addresses, ports, and protocols. It does not inspect application-layer data (Layer 7), which matches the scenario where traffic is allowed based on source IP address only. This is the simplest and fastest type of firewall, but it lacks the ability to block attacks embedded in application payloads.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
This CISSP practice question is part of Courseiva's free ISC2 certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CISSP exam.
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