The correct answer is to allow traffic from the outside interface to the DMZ interface on port 80. This rule is necessary because external users on the Internet must reach the web server hosted in the DMZ, and the firewall must explicitly permit inbound HTTP requests (TCP port 80) from the untrusted outside zone to the isolated DMZ zone. The core security concept here is that the DMZ acts as a buffer zone, allowing public access to the web server while keeping the corporate inside network completely separate and protected from direct external traffic. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of multi-homed firewall architectures and zone-based policies. A common trap is selecting a rule that allows traffic from the outside to the inside interface, which would expose the corporate LAN. Remember the DMZ mantra: outside to DMZ is allowed for services, but outside to inside is always blocked. A helpful memory tip is "DMZ is the middle ground"—the firewall rule should never bridge the outside directly to the inside.
N10-009 Network Security Practice Question
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 hosts a web server in a DMZ. The firewall has three interfaces: inside (corporate network), outside (Internet), and DMZ. Which firewall rule is necessary to allow external users to access the web server?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Allow traffic from the outside interface to the DMZ interface on port 80
Option A is correct because external users on the Internet (outside interface) need to reach the web server located in the DMZ. The firewall must permit inbound traffic from the outside zone to the DMZ zone on TCP port 80 (HTTP) to allow web requests while keeping the corporate network (inside) isolated from direct external access.
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.
✓
Allow traffic from the outside interface to the DMZ interface on port 80
Why this is correct
This rule permits external traffic to reach the web server in the DMZ while keeping the internal network isolated.
Related concept
Read the scenario before looking for a memorised answer.
✗
Allow traffic from the inside interface to the DMZ interface on port 80
Why it's wrong here
This rule permits internal users, not external users, to access the web server.
✗
Allow traffic from the outside interface to the inside interface on port 80
Why it's wrong here
This would expose the internal network to direct inbound traffic, defeating the purpose of a DMZ.
✗
Deny all traffic by default and create no specific rules
Why it's wrong here
A default deny without an explicit allow rule would block all traffic, including legitimate web requests.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates mistakenly choose Option B or C, confusing the direction of traffic or thinking that external users need access to the inside network, when the correct security design is to restrict external traffic only to the DMZ.
Detailed technical explanation
How to think about this question
In a three-legged firewall architecture, each interface belongs to a separate security zone, and traffic between zones is controlled by stateful inspection rules. The DMZ acts as a buffer zone where public-facing servers reside; allowing only necessary ports (e.g., TCP 80/443) from outside to DMZ ensures that even if the web server is compromised, the attacker cannot pivot directly to the inside network. Real-world deployments often add a rule to allow return traffic (established connections) and may use NAT to map a public IP to the DMZ server.
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
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: Allow traffic from the outside interface to the DMZ interface on port 80 — Option A is correct because external users on the Internet (outside interface) need to reach the web server located in the DMZ. The firewall must permit inbound traffic from the outside zone to the DMZ zone on TCP port 80 (HTTP) to allow web requests while keeping the corporate network (inside) isolated from direct external access.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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