The answer is that the VPC Flow Log ACCEPT entry represents return traffic from an outbound connection initiated by the instance. This is because VPC Flow Logs are stateful—they record the entire connection context, not individual packets. When an instance at 10.0.1.5 initiates an outbound TCP connection to 203.0.113.50 on port 443, the flow log captures the return path as an ACCEPT from the internet host back to the instance, even though the security group’s inbound rule only permits traffic from 10.0.0.0/16. The security group’s default outbound rule allows all traffic, and the subnet’s outbound NACL (not shown) also permits it, so the return ACCEPT is valid. On the ANS-C01 exam, this tests your understanding of stateful versus stateless filtering and how flow logs mirror connection state, not packet direction. A common trap is misreading the source and destination IPs in the log as the direction of traffic flow. Memory tip: “Flow logs follow the conversation, not the packet—always check which side initiated the TCP handshake.”
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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.
VPC Flow Logs entry:
2 123456789010 eni-12345678 10.0.1.5 203.0.113.50 443 38000 6 20 5000 1450670868 1450670868 ACCEPT OK
And the following security group inbound rule:
Type: Custom TCP, Protocol: TCP, Port Range: 443, Source: 10.0.0.0/16
And NACL inbound rule:
Rule #100: Type: HTTP (80), Protocol: TCP, Port Range: 80, Source: 0.0.0.0/0, Allow
Rule #120: Type: HTTPS (443), Protocol: TCP, Port Range: 443, Source: 10.0.0.0/16, Allow
Rule #*: Type: All traffic, Protocol: All, Port Range: All, Source: 0.0.0.0/0, Deny
A network engineer analyzes a VPC Flow Log entry showing an ACCEPT for a TCP connection from 203.0.113.50 (internet) to 10.0.1.5 on port 443. The security group for the instance allows inbound HTTPS only from 10.0.0.0/16, and the NACL for the subnet has the rules shown. Why was the traffic accepted?
Refer to the exhibit.
VPC Flow Logs entry:
2 123456789010 eni-12345678 10.0.1.5 203.0.113.50 443 38000 6 20 5000 1450670868 1450670868 ACCEPT OK
And the following security group inbound rule:
Type: Custom TCP, Protocol: TCP, Port Range: 443, Source: 10.0.0.0/16
And NACL inbound rule:
Rule #100: Type: HTTP (80), Protocol: TCP, Port Range: 80, Source: 0.0.0.0/0, Allow
Rule #120: Type: HTTPS (443), Protocol: TCP, Port Range: 443, Source: 10.0.0.0/16, Allow
Rule #*: Type: All traffic, Protocol: All, Port Range: All, Source: 0.0.0.0/0, Deny
A
The NACL inbound rule #120 allows HTTPS from 10.0.0.0/16, but the source is 203.0.113.50, so it should be denied.
Why wrong: Same as above.
B
The NACL inbound rule #100 allows HTTP from anywhere, and the traffic is HTTP.
Why wrong: The traffic is HTTPS (port 443), not HTTP.
C
The security group allows inbound HTTPS from 10.0.0.0/16, but the flow log source is 203.0.113.50, so it should be denied. The ACCEPT must be an error.
Why wrong: Flow logs are accurate; the traffic was accepted because it was outbound from the instance.
D
The flow log entry represents outbound traffic from the instance (source 10.0.1.5:443 to destination 203.0.113.50:38000). The security group's default outbound rule allows all traffic, and the NACL outbound (not shown) also allows it, resulting in ACCEPT.
Correct interpretation of the flow log: source is the instance, destination is internet. Outbound is allowed by default.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The flow log entry represents outbound traffic from the instance (source 10.0.1.5:443 to destination 203.0.113.50:38000). The security group's default outbound rule allows all traffic, and the NACL outbound (not shown) also allows it, resulting in ACCEPT.
The VPC Flow Log entry shows an ACCEPT for traffic from 203.0.113.50:38000 to 10.0.1.5:443. Because flow logs record connection state, this entry actually represents the return traffic of an outbound connection initiated by the instance (10.0.1.5) to the internet host on port 443. The security group's default outbound rule allows all traffic, and the NACL outbound rules (not shown) also permit it, so the return ACCEPT is valid.
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 rule #120 allows HTTPS from 10.0.0.0/16, but the source is 203.0.113.50, so it should be denied.
Why it's wrong here
Same as above.
✗
The NACL inbound rule #100 allows HTTP from anywhere, and the traffic is HTTP.
The security group allows inbound HTTPS from 10.0.0.0/16, but the flow log source is 203.0.113.50, so it should be denied. The ACCEPT must be an error.
Why it's wrong here
Flow logs are accurate; the traffic was accepted because it was outbound from the instance.
✓
The flow log entry represents outbound traffic from the instance (source 10.0.1.5:443 to destination 203.0.113.50:38000). The security group's default outbound rule allows all traffic, and the NACL outbound (not shown) also allows it, resulting in ACCEPT.
Why this is correct
Correct interpretation of the flow log: source is the instance, destination is internet. Outbound is allowed by default.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the misconception that VPC Flow Log entries always represent the direction of traffic as seen by the network, when in fact they record the connection tuple as observed, which can be the reverse direction of the original request, leading candidates to incorrectly apply inbound security rules.
Detailed technical explanation
How to think about this question
VPC Flow Logs capture connection-level metadata and record both directions of a flow as separate log entries, but the ACCEPT/REJECT status reflects the stateful nature of the connection. For outbound traffic, the security group's stateful engine automatically allows return traffic regardless of inbound rules, while NACLs are stateless and require explicit outbound rules for the response. In this scenario, the instance initiated a connection to 203.0.113.50 on port 443, and the flow log entry showing source 10.0.1.5:443 and destination 203.0.113.50:38000 is the return path.
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 Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The flow log entry represents outbound traffic from the instance (source 10.0.1.5:443 to destination 203.0.113.50:38000). The security group's default outbound rule allows all traffic, and the NACL outbound (not shown) also allows it, resulting in ACCEPT. — The VPC Flow Log entry shows an ACCEPT for traffic from 203.0.113.50:38000 to 10.0.1.5:443. Because flow logs record connection state, this entry actually represents the return traffic of an outbound connection initiated by the instance (10.0.1.5) to the internet host on port 443. The security group's default outbound rule allows all traffic, and the NACL outbound rules (not shown) also permit it, so the return ACCEPT is valid.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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