The answer is a security group rule blocking the return traffic. When troubleshooting VPC Flow Logs that show ACCEPT then REJECT for the same flow, the first packet is accepted by the stateful security group, but the return packet is dropped because the security group does not have an inbound rule allowing the ephemeral port range for the response. This occurs because security groups are stateful only for traffic they initiate; if the return traffic does not match an existing session or an explicit inbound rule, it is rejected. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how stateful security groups differ from stateless network ACLs—a common trap is confusing the two or assuming a routing issue. Remember the memory tip: “First ACCEPT, then REJECT? Check the security group’s return path, not the route.”
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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.
CLI output from a VPC Flow Log:
2 123456789010 eni-12345 10.0.1.5 10.0.2.10 443 54872 6 10 1000 1432919027 1432919028 ACCEPT OK
2 123456789010 eni-12345 10.0.1.5 10.0.2.10 443 54873 6 25 4000 1432919028 1432919029 REJECT OK
An engineer is reviewing VPC Flow Logs for connectivity issues between two EC2 instances (10.0.1.5 and 10.0.2.10) on TCP port 443. The first log entry shows ACCEPT, the second shows REJECT. What is the most likely cause of the REJECT?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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
✓
A security group rule is blocking the return traffic.
Option A is correct because the REJECT indicates that the packet was dropped by a security group or NACL. Since the first packet was accepted, a stateful security group might be blocking the return traffic if the session is not tracked. Option B is incorrect because REJECT is not due to routing. Option C is incorrect because the log shows the traffic as OK. Option D is incorrect because Flow Logs do not indicate interface issues.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 destination instance has an incorrect network interface configuration.
Why it's wrong here
Interface misconfiguration would not produce a REJECT in Flow Logs.
✗
The network ACL is blocking the traffic due to an incorrect inbound rule.
Why it's wrong here
NACLs are stateless; they would block both directions if misconfigured.
✗
The route table for the subnet does not have a route to the destination.
Why it's wrong here
Routing issues would result in no log entry or a different status.
✓
A security group rule is blocking the return traffic.
Why this is correct
Security groups are stateful and if the return traffic does not match the allowed outbound rule, it can be rejected.
Clue confirmation
The clue words "first", "most likely" in the question point toward this answer.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: A security group rule is blocking the return traffic. — Option A is correct because the REJECT indicates that the packet was dropped by a security group or NACL. Since the first packet was accepted, a stateful security group might be blocking the return traffic if the session is not tracked. Option B is incorrect because REJECT is not due to routing. Option C is incorrect because the log shows the traffic as OK. Option D is incorrect because Flow Logs do not indicate interface issues.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "first", "most likely". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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