The answer is that the ACL is missing a permit rule for the source IP 203.0.113.5. This is correct because access control lists process rules sequentially from top to bottom, and if no explicit permit statement matches the user’s traffic to the internal web server at 10.0.0.10 over HTTPS (TCP/443), the implicit deny rule at the end of the ACL will drop the packet. On the CISSP exam, this scenario tests your understanding of ACL mechanics and the principle of default-deny—a core concept in network security architecture. A common trap is assuming a missing rule means the traffic is allowed; in reality, the absence of a permit is the direct cause of denial. Remember the mnemonic “No permit, no packet”—if you don’t explicitly say yes, the ACL says no.
CISSP Communication and Network Security Practice Question
This CISSP practice question tests your understanding of communication and 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.
Exhibit
Refer to the exhibit. The following firewall log entry shows a denied packet:
Deny tcp 203.0.113.5 52314 10.0.0.10 443
The firewall has the following ACL applied inbound on the external interface:
ip access-list extended INSIDE-IN
permit tcp host 203.0.113.2 host 10.0.0.10 eq 443
deny ip any any log
A remote user at 203.0.113.5 cannot access the internal web server at 10.0.0.10 over HTTPS. What is the most likely cause of the denial?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Refer to the exhibit. The following firewall log entry shows a denied packet:
Deny tcp 203.0.113.5 52314 10.0.0.10 443
The firewall has the following ACL applied inbound on the external interface:
ip access-list extended INSIDE-IN
permit tcp host 203.0.113.2 host 10.0.0.10 eq 443
deny ip any any log
A
The ACL is missing a permit rule for the user's IP
The permit rule only allows 203.0.113.2, so traffic from 203.0.113.5 is denied.
B
The ACL is applied in the wrong direction
Why wrong: Inbound on the external interface is correct for traffic coming from the internet.
C
The firewall is not performing stateful inspection
Why wrong: Stateful inspection would not change the ACL matching; the packet is statically denied.
D
The web server is not listening on port 443
Why wrong: The log shows a deny, not a connection refused; the firewall is blocking.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ACL is missing a permit rule for the user's IP
The user at 203.0.113.5 is attempting to reach the internal web server at 10.0.0.10 over HTTPS (TCP/443). If an ACL is applied on the firewall or router interface that filters inbound traffic from the remote user, the most direct cause of denial is the absence of a permit rule for the source IP 203.0.113.5. ACLs process rules sequentially, and if no explicit permit matches the user's traffic, the implicit deny at the end of the ACL will drop the packet.
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.
✓
The ACL is missing a permit rule for the user's IP
Why this is correct
The permit rule only allows 203.0.113.2, so traffic from 203.0.113.5 is denied.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The ACL is applied in the wrong direction
Why it's wrong here
Inbound on the external interface is correct for traffic coming from the internet.
✗
The firewall is not performing stateful inspection
Why it's wrong here
Stateful inspection would not change the ACL matching; the packet is statically denied.
✗
The web server is not listening on port 443
Why it's wrong here
The log shows a deny, not a connection refused; the firewall is blocking.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse ACL directionality (Option B) with the actual missing rule, but the symptom of a complete denial for a specific source IP points directly to a missing permit entry rather than a directional issue.
Trap categories for this question
Command / output trap
The log shows a deny, not a connection refused; the firewall is blocking.
Detailed technical explanation
How to think about this question
ACLs on Cisco routers and firewalls use an implicit deny all at the end of the list, meaning any traffic not explicitly permitted is dropped. In this scenario, the remote user's IP (203.0.113.5) must be matched by a permit statement for TCP port 443 to the destination 10.0.0.10. A common real-world misconfiguration is forgetting to add the permit rule after changing the ACL, resulting in a sudden denial of service for legitimate users.
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 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Communication and Network Security — This question tests Communication and Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ACL is missing a permit rule for the user's IP — The user at 203.0.113.5 is attempting to reach the internal web server at 10.0.0.10 over HTTPS (TCP/443). If an ACL is applied on the firewall or router interface that filters inbound traffic from the remote user, the most direct cause of denial is the absence of a permit rule for the source IP 203.0.113.5. ACLs process rules sequentially, and if no explicit permit matches the user's traffic, the implicit deny at the end of the ACL will drop the packet.
What should I do if I get this CISSP 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: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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