The answer is that the subnet’s NACL inbound rules are blocking the return traffic on ephemeral ports. This occurs because a Network ACL is stateless, meaning it evaluates inbound and outbound traffic independently. While the outbound rule allows traffic on port 443, the return packets from 203.0.113.50 back to the EC2 instance use a random ephemeral port (typically 1024–65535) as the destination. Without an explicit inbound NACL rule permitting this ephemeral port range, the return traffic is dropped, causing VPC Flow Logs to show the outbound connection as rejected. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of stateless versus stateful filtering and the common trap of only checking outbound rules. A reliable memory tip: “NACLs are stateless—for every out, you need an in on ephemeral ports.”
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 has a VPC with a CIDR of 10.0.0.0/16 and has enabled VPC Flow Logs to capture all traffic. The logs show that an EC2 instance (10.0.1.10) is sending outbound traffic to an external IP (203.0.113.50) on port 443, but the traffic is being rejected. The instance's security group allows outbound HTTPS to 0.0.0.0/0, and the subnet's NACL allows outbound traffic on port 443. The VPC has an internet gateway attached, and the route table directs 0.0.0.0/0 to the internet gateway. What is the most likely cause of the rejection?
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 NACL inbound rules are blocking the return traffic.
The outbound traffic from the EC2 instance is allowed by the security group and NACL outbound rules, but the return traffic (responses from 203.0.113.50:443 back to 10.0.1.10) must also be permitted by the subnet's NACL inbound rules. Since the NACL is stateless, it requires explicit inbound rules for ephemeral ports (typically 1024-65535) to allow the return traffic. If those inbound rules are missing, the return packets are dropped, causing the outbound traffic to appear as rejected in VPC Flow Logs.
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 NACL inbound rules are blocking the return traffic.
Why this is correct
NACLs are stateless and require explicit inbound rules for return traffic.
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 security group does not allow inbound HTTPS traffic.
Why it's wrong here
Security groups are stateful; outbound allowance implies inbound return traffic is allowed.
✗
The internet gateway is not attached to the VPC.
Why it's wrong here
It is attached.
✗
The route table does not have a route to the internet gateway.
Why it's wrong here
Route is present.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the distinction between stateful security groups and stateless NACLs, specifically that NACLs require explicit inbound rules for return traffic while security groups automatically allow it.
Detailed technical explanation
How to think about this question
NACLs are stateless, meaning inbound and outbound rules are evaluated independently. For outbound traffic to an external HTTPS server, the NACL must allow outbound traffic on destination port 443 (which it does) and also allow inbound traffic on the ephemeral source port (typically 1024-65535) that the EC2 instance uses for the connection. VPC Flow Logs record the 'reject' status when the packet is dropped by the NACL or security group; in this case, the missing inbound NACL rule causes the drop. This is a common misconfiguration when using NACLs with internet-bound traffic.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The NACL inbound rules are blocking the return traffic. — The outbound traffic from the EC2 instance is allowed by the security group and NACL outbound rules, but the return traffic (responses from 203.0.113.50:443 back to 10.0.1.10) must also be permitted by the subnet's NACL inbound rules. Since the NACL is stateless, it requires explicit inbound rules for ephemeral ports (typically 1024-65535) to allow the return traffic. If those inbound rules are missing, the return packets are dropped, causing the outbound traffic to appear as rejected in VPC Flow Logs.
What should I do if I get this ANS-C01 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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