The answer is ACL misordering, specifically a deny statement placed before a permit rule. This is the most likely cause because access control lists are processed top-down, and the first matching rule is applied; a 'deny ip any any' entry will match all traffic before a subsequent 'permit tcp host 10.0.0.10 eq 80' rule can ever be evaluated, effectively blocking all packets regardless of the intended destination. On the CompTIA SecurityX CAS-004 exam, this scenario tests your understanding of ACL evaluation order and common misconfiguration pitfalls, often appearing in troubleshooting questions where traffic unexpectedly fails. A common trap is assuming the permit rule will override the deny, but the sequential nature of ACLs means the first match wins. Remember the memory tip: "First match is final—deny before permit leaves no room for traffic."
CAS-004 Security Architecture Practice Question
This CAS-004 practice question tests your understanding of security architecture. 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.
Exhibit
access-list 100 deny ip any any
access-list 100 permit tcp any host 10.0.0.10 eq 80
Refer to the exhibit. A security analyst notices that traffic from external clients to the web server at 10.0.0.10 port 80 is being blocked. Which of the following is the MOST likely cause?
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.
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 misordered (deny before permit)
The ACL is processed top-down. The 'deny ip any any' rule matches all traffic before the permit rule, so all traffic is blocked. Misordering is the cause. The web server using HTTPS (443) is possible but not indicated, and the destination IP appears correct. Lack of logging is not the cause.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 lacks a log statement
Why it's wrong here
Logging is optional and does not affect permit/deny behavior.
✓
The ACL is misordered (deny before permit)
Why this is correct
The deny all rule (line 1) blocks all traffic before the permit rule is evaluated.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The web server is using port 443
Why it's wrong here
The ACL allows port 80; if the server used 443, traffic would be implicitly denied, but the exhibit shows a port 80 rule.
✗
The destination IP is incorrect
Why it's wrong here
The destination IP matches the web server, so this is not the issue.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Trap categories for this question
Command / output trap
The ACL allows port 80; if the server used 443, traffic would be implicitly denied, but the exhibit shows a port 80 rule.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related CAS-004 NAT questions on configuration and troubleshooting.
Security Architecture — This question tests Security Architecture — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The ACL is misordered (deny before permit) — The ACL is processed top-down. The 'deny ip any any' rule matches all traffic before the permit rule, so all traffic is blocked. Misordering is the cause. The web server using HTTPS (443) is possible but not indicated, and the destination IP appears correct. Lack of logging is not the cause.
What should I do if I get this CAS-004 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related CAS-004 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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