The answer is the ACL’s implicit deny rule blocking HTTPS management traffic. When an engineer configures an IPv4 ACL to permit only HTTP (TCP port 80) from a specific subnet and applies it inbound on a router interface, any traffic not explicitly matched—such as HTTPS (TCP port 443)—is denied by the implicit deny any at the end of every standard or extended ACL. This is why the router’s web interface becomes unreachable from that subnet, even though HTTP is allowed. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how ACLs filter control plane traffic and the common trap of forgetting that management protocols like HTTPS, SSH, or SNMP require separate permit entries. A reliable memory tip is “permit what you need, deny the rest—implicit deny never rests.”
300-410 IPv4 Access Control Lists Practice Question
This 300-410 practice question tests your understanding of ipv4 access control lists. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
An engineer configures an IPv4 ACL on a router's interface to permit only HTTP traffic (TCP port 80) from a specific subnet. The ACL is applied inbound. After applying, the router's web interface (HTTPS) becomes unreachable from the same subnet. What is the most likely explanation?
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 denies HTTPS traffic because it is not explicitly permitted.
The ACL is applied inbound on the router interface and only permits TCP port 80 (HTTP). HTTPS uses TCP port 443, which is not explicitly permitted. Since IPv4 ACLs end with an implicit deny any, all traffic not matching a permit statement, including HTTPS, is denied. This causes the router's web interface (HTTPS) to become unreachable from the subnet.
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 denies HTTPS traffic because it is not explicitly permitted.
Why this is correct
The ACL only permits HTTP; HTTPS (port 443) is denied by the implicit deny, blocking management access.
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 outbound, filtering traffic to the web server.
Why it's wrong here
Outbound ACL would affect outgoing traffic, not incoming management.
✗
The router's web server uses HTTP, not HTTPS.
Why it's wrong here
Modern routers use HTTPS by default; HTTP is often disabled.
Common exam trap: answer the scenario, not the keyword
Cisco often tests the implicit deny any behavior of ACLs and the fact that management protocols (like HTTPS, SSH, SNMP) use different ports than the permitted traffic, causing candidates to overlook the need to explicitly permit those ports.
Detailed technical explanation
How to think about this question
Cisco IOS ACLs have an implicit deny any at the end, meaning any traffic not explicitly matched by a permit statement is dropped. When an ACL is applied inbound, it filters traffic before the router processes it locally, so management protocols like HTTPS (TCP 443) or SSH (TCP 22) must be explicitly permitted if they originate from the same subnet. In real-world scenarios, engineers often forget to permit management traffic when applying restrictive ACLs to router interfaces, leading to loss of remote access.
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 300-410 question in full detail.
IPv4 Access Control Lists — This question tests IPv4 Access Control Lists — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ACL denies HTTPS traffic because it is not explicitly permitted. — The ACL is applied inbound on the router interface and only permits TCP port 80 (HTTP). HTTPS uses TCP port 443, which is not explicitly permitted. Since IPv4 ACLs end with an implicit deny any, all traffic not matching a permit statement, including HTTPS, is denied. This causes the router's web interface (HTTPS) to become unreachable from the subnet.
What should I do if I get this 300-410 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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Question Discussion
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