The answer is stateful inspection, because it tracks the state of active connections by maintaining a state table that records source and destination IPs, ports, and sequence numbers. This technology allows return traffic for connections initiated from the internal network while blocking unsolicited inbound traffic, directly fulfilling the requirement that external hosts cannot initiate connections unless the internal host requested them first. On the CompTIA Network+ N10-009 exam, this concept tests your understanding of how stateful firewall inspection allow return traffic as part of the core security mechanisms, often appearing in scenario-based questions where you must distinguish it from stateless packet filtering or application-layer gateways. A common trap is confusing stateful inspection with a simple access control list—remember that stateful firewalls dynamically create temporary permit entries for return traffic based on the session table. Memory tip: think of it as a “conversation tracker”—if your internal host starts the chat, the firewall lets the reply back in, but it slams the door on uninvited guests.
N10-009 Network Security Practice Question
This N10-009 practice question tests your understanding of network security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
A security engineer is configuring a firewall to protect an internal network. The requirement is that internal users can initiate connections to the internet, but external hosts should not be able to initiate connections to internal hosts unless the internal host first requested the connection. Which firewall technology should be used?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Why wrong: Stateless firewalls filter each packet independently without considering connection state, making them unable to allow only return traffic.
B
Stateful inspection
Stateful firewalls track the state of connections and permit inbound packets only if they match an existing session.
C
Application proxy
Why wrong: Application proxy acts as an intermediary and can filter at the application layer, but it is not the primary technology for stateful traffic control.
D
Packet filtering based on ACL only
Why wrong: ACL-based filtering is stateless and would require explicit rules for return traffic, which is less efficient.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Stateful inspection
Stateful inspection (B) tracks the state of active connections by maintaining a state table that records source/destination IPs, ports, and sequence numbers. It allows return traffic for connections initiated from the internal network while blocking unsolicited inbound traffic, which directly matches the requirement that external hosts cannot initiate connections unless the internal host requested them first.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Stateless packet filtering
Why it's wrong here
Stateless firewalls filter each packet independently without considering connection state, making them unable to allow only return traffic.
✓
Stateful inspection
Why this is correct
Stateful firewalls track the state of connections and permit inbound packets only if they match an existing session.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Application proxy
Why it's wrong here
Application proxy acts as an intermediary and can filter at the application layer, but it is not the primary technology for stateful traffic control.
✗
Packet filtering based on ACL only
Why it's wrong here
ACL-based filtering is stateless and would require explicit rules for return traffic, which is less efficient.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that stateless packet filtering can handle return traffic by simply allowing inbound packets with a high source port, but without state tracking, it cannot verify that the packet actually belongs to an existing session, making stateful inspection the correct answer.
Detailed technical explanation
How to think about this question
Stateful inspection uses a state table that tracks TCP connection states (SYN, SYN-ACK, ACK) and UDP pseudo-state information (e.g., timeout-based session tracking). For TCP, it validates that inbound packets have the ACK flag set and match an existing session entry, dropping packets with only the SYN flag that are not part of a known handshake. In real-world deployments, this technology is implemented in firewalls like Cisco ASA or pfSense, where the state table size and timeout values (e.g., TCP idle timeout of 1 hour) directly impact performance and security.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
Network Security — This question tests Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Stateful inspection — Stateful inspection (B) tracks the state of active connections by maintaining a state table that records source/destination IPs, ports, and sequence numbers. It allows return traffic for connections initiated from the internal network while blocking unsolicited inbound traffic, which directly matches the requirement that external hosts cannot initiate connections unless the internal host requested them first.
What should I do if I get this N10-009 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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