The answer is a security group on the destination instance that allows inbound HTTPS but denies outbound traffic. This configuration causes intermittent reject in VPC Flow Logs because security groups are stateful—when the inbound rule permits the initial HTTPS request, the return traffic is expected to be automatically allowed, but if the outbound rule explicitly denies all traffic, the response packets are dropped, leading to a mix of ACCEPT and REJECT entries. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the critical difference between stateful security groups and stateless network ACLs; a common trap is assuming NACLs cause intermittent behavior, but they would block consistently, not intermittently. Remember the memory tip: “Stateful groups track sessions, but an explicit deny breaks the return path.”
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.
Exhibit
Refer to the exhibit. The following is an excerpt from a VPC Flow Log:
2 123456789010 eni-12345678 10.0.1.5 10.0.2.10 443 12345 6 10 1000 1234567890 1234567890 ACCEPT OK
2 123456789010 eni-12345678 10.0.1.5 10.0.2.10 443 12346 6 10 1000 1234567890 1234567890 REJECT OK
A network engineer is reviewing VPC Flow Logs for a VPC. The logs show that traffic from 10.0.1.5 to 10.0.2.10 on port 443 is being accepted and rejected intermittently. Both instances are in the same VPC. 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.
Refer to the exhibit. The following is an excerpt from a VPC Flow Log:
2 123456789010 eni-12345678 10.0.1.5 10.0.2.10 443 12345 6 10 1000 1234567890 1234567890 ACCEPT OK
2 123456789010 eni-12345678 10.0.1.5 10.0.2.10 443 12346 6 10 1000 1234567890 1234567890 REJECT OK
A
The security group on the destination instance is configured to allow inbound HTTPS but deny outbound traffic.
Security groups are stateful; if outbound is denied, return traffic is rejected.
B
The network ACL is blocking traffic.
Why wrong: NACLs are stateless and would block all or none, not intermittently.
C
The route table is misconfigured.
Why wrong: Route tables don't cause intermittent accepts/rejects.
D
The source instance's security group is denying outbound traffic.
Why wrong: If source denies outbound, no traffic would be sent.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The security group on the destination instance is configured to allow inbound HTTPS but deny outbound traffic.
Option C is correct because a security group that allows inbound HTTPS but denies outbound can cause intermittent rejects if the return traffic is blocked. NACLs are stateless and would block consistently. B and D are less likely.
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 security group on the destination instance is configured to allow inbound HTTPS but deny outbound traffic.
Why this is correct
Security groups are stateful; if outbound is denied, return traffic is rejected.
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 network ACL is blocking traffic.
Why it's wrong here
NACLs are stateless and would block all or none, not intermittently.
✗
The route table is misconfigured.
Why it's wrong here
Route tables don't cause intermittent accepts/rejects.
✗
The source instance's security group is denying outbound traffic.
Why it's wrong here
If source denies outbound, no traffic would be sent.
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.
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 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.
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 ANS-C01 NAT questions on configuration and troubleshooting.
Network Implementation — This question tests Network Implementation — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The security group on the destination instance is configured to allow inbound HTTPS but deny outbound traffic. — Option C is correct because a security group that allows inbound HTTPS but denies outbound can cause intermittent rejects if the return traffic is blocked. NACLs are stateless and would block consistently. B and D are less likely.
What should I do if I get this ANS-C01 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 ANS-C01 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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