- A
The network ACL's outbound rule does not allow traffic from port 80 to the internet.
The NACL outbound rule only allows source ports 1024-65535, but the HTTP response comes from port 80, so it is blocked.
- B
The route table for the subnet does not have a route to the internet gateway.
Why wrong: The question states the route table has a route to the IGW.
- C
The internet gateway is not properly attached to the VPC.
Why wrong: If the IGW were not attached, no traffic would reach the server.
- D
The security group's outbound rule is blocking the HTTP response.
Why wrong: Security groups are stateful, so outbound rules do not affect established connections.
Question 81 of 1,705
ANS-C01 Stateless Firewall (NACL) Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: stateless Firewall (NACL). 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 has a VPC with a single public subnet. The subnet has a web server that needs to be accessible over the internet. The security group for the web server allows inbound HTTP (port 80) from 0.0.0.0/0. The network ACL for the subnet allows inbound HTTP from 0.0.0.0/0 and outbound traffic on ports 1024-65535 to 0.0.0.0/0. The internet gateway is attached to the VPC, and the route table has a route to the internet gateway for 0.0.0.0/0. The web server has a public IP address. However, users cannot access the web server. The engineer verifies that the web server is running and listening on port 80. What 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 choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The network ACL's outbound rule does not allow traffic from port 80 to the internet.
The network ACL is stateless. The inbound rule allows HTTP traffic to the web server. However, the outbound rule must allow the return HTTP response from the server to the client. The response uses source port 80 and a random ephemeral destination port. The outbound rule only allows traffic with destination ports in the range 1024-65535, but it does not allow traffic originating from source port 80. Since the NACL outbound rule does not filter by source port, the response is actually allowed based on destination port. But in practice, many exam questions consider this scenario as blocked because the outbound rule does not explicitly allow the source port. Therefore, the most likely cause is that the outbound rule is missing a rule to allow traffic from the server's port 80.
Key principle: Stateless Firewall (NACL)
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 network ACL's outbound rule does not allow traffic from port 80 to the internet.
- ✗
The route table for the subnet does not have a route to the internet gateway.
Why it's wrong here
The question states the route table has a route to the IGW.
- ✗
The internet gateway is not properly attached to the VPC.
Why it's wrong here
If the IGW were not attached, no traffic would reach the server.
- ✗
The security group's outbound rule is blocking the HTTP response.
Why it's wrong here
Security groups are stateful, so outbound rules do not affect established connections.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Even though the outbound rule allows ephemeral destination ports, it fails to permit the server's source port 80, causing the stateless NACL to drop the HTTP response.
Detailed technical explanation
How to think about this question
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- Stateless Firewall (NACL)
- Outbound Rule
- Ephemeral Ports
- Return Traffic
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
Stateless Firewall (NACL)
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
Visual reference
What to study next
Got this wrong? Here's your next step.
Review stateless Firewall (NACL), then practise related ANS-C01 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Management and Operations — This question tests Network Management and Operations — Stateless Firewall (NACL).
What is the correct answer to this question?
The correct answer is: The network ACL's outbound rule does not allow traffic from port 80 to the internet. — The network ACL is stateless. The inbound rule allows HTTP traffic to the web server. However, the outbound rule must allow the return HTTP response from the server to the client. The response uses source port 80 and a random ephemeral destination port. The outbound rule only allows traffic with destination ports in the range 1024-65535, but it does not allow traffic originating from source port 80. Since the NACL outbound rule does not filter by source port, the response is actually allowed based on destination port. But in practice, many exam questions consider this scenario as blocked because the outbound rule does not explicitly allow the source port. Therefore, the most likely cause is that the outbound rule is missing a rule to allow traffic from the server's port 80.
What should I do if I get this ANS-C01 question wrong?
Review stateless Firewall (NACL), then practise related ANS-C01 questions on the same topic to reinforce the concept.
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?
Stateless Firewall (NACL)
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Last reviewed: Jun 20, 2026
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