Quick Answer
The answer is to permit inbound HTTP/HTTPS traffic from the outside interface to the DMZ interface, typically destined for the web server’s IP on ports 80 and 443. This is correct because a three-legged firewall setup requires a specific rule that allows external users to reach the DMZ while explicitly blocking any direct traffic from the outside to the inside network, thereby isolating the internal LAN from untrusted access. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of firewall rule placement and the principle of least privilege—a common trap is confusing the source and destination interfaces, such as allowing traffic from the DMZ to the outside instead. Remember that the DMZ acts as a buffer zone: external traffic must be permitted to the DMZ only, never directly to the inside. A helpful memory tip is “Outside to DMZ, never inside,” reinforcing that the rule’s source is the outside interface and its destination is the DMZ interface.
This N10-009 practice question tests your understanding of 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.
A company wants to allow external users to access a web server located in the DMZ. The firewall has three interfaces: inside, outside, and DMZ. Which firewall rule is necessary?
A
Why wrong: Allowing from inside to DMZ would permit internal users to access the web server, not external users.
B
This rule permits external users to initiate connections to the web server in the DMZ on port 80.
C
Why wrong: Allowing from DMZ to inside is not needed for external access and could pose a security risk.
D
Why wrong: A deny rule from outside to inside would block all incoming traffic, defeating the purpose.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
B
Option B is correct because it allows traffic from the outside (external users) to the DMZ web server while keeping the inside network protected. In a three-legged firewall setup, the necessary rule permits inbound HTTP/HTTPS traffic from the outside interface to the DMZ interface, typically with a destination IP of the web server and port 80/443. This ensures external access is isolated from the internal network, adhering to the principle of least privilege.
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.
A
Why it's wrong here
Allowing from inside to DMZ would permit internal users to access the web server, not external users.
B
Why this is correct
This rule permits external users to initiate connections to the web server in the DMZ on port 80.
Related concept
Read the scenario before looking for a memorised answer.
C
Why it's wrong here
Allowing from DMZ to inside is not needed for external access and could pose a security risk.
D
Why it's wrong here
A deny rule from outside to inside would block all incoming traffic, defeating the purpose.
Common exam traps
The trap here is that candidates often confuse the direction of traffic flow, mistakenly selecting a rule that allows inside-to-DMZ traffic (Option A) thinking it enables external access, when in fact external users initiate from the outside interface.
Detailed technical explanation
In a three-interface firewall (inside, outside, DMZ), stateful inspection tracks connection states; the rule for outside-to-DMZ must be paired with a corresponding implicit or explicit rule allowing return traffic. Real-world scenarios often require additional rules for NAT (e.g., static NAT mapping the web server’s public IP to its private DMZ IP) and application-layer inspection (e.g., for HTTP/HTTPS) to prevent protocol anomalies. The DMZ acts as a buffer zone, so traffic from the DMZ to the inside is typically blocked by default to contain breaches.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
What to study next
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FAQ
Network Security — This question tests Network Security — Read the scenario before looking for a memorised answer..
The correct answer is: B — Option B is correct because it allows traffic from the outside (external users) to the DMZ web server while keeping the inside network protected. In a three-legged firewall setup, the necessary rule permits inbound HTTP/HTTPS traffic from the outside interface to the DMZ interface, typically with a destination IP of the web server and port 80/443. This ensures external access is isolated from the internal network, adhering to the principle of least privilege.
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 11, 2026
This N10-009 practice question is part of Courseiva's free CompTIA 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 N10-009 exam.
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