The answer is that the database security group does not allow outbound traffic for the response, but this is actually a trick—the real issue is that security group stateful outbound rules troubleshooting often reveals a misunderstanding of statefulness. Security groups are stateful, meaning that if inbound traffic on port 3306 is allowed from the application security group, the return traffic is automatically permitted regardless of outbound rules; therefore, the database security group’s outbound rules are not the cause. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your grasp of stateful versus stateless filtering, a common trap where candidates incorrectly blame outbound rules when the real culprit is often a missing inbound rule or a misconfigured network ACL. Remember the memory tip: “Stateful means return traffic is a free pass—only stateless NACLs need explicit outbound rules for responses.”
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 production VPC with a public subnet and a private subnet. The private subnet hosts a database instance that should be accessible only from the application servers in the same VPC. The security team has configured the database security group to allow inbound traffic on port 3306 from the application security group. However, the application servers cannot connect to the database. The network ACLs are configured with default allow all rules. 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.
The database instance does not have an IAM role assigned.
Why wrong: Wrong: IAM not needed for connectivity.
B
The database security group does not allow outbound traffic for the response.
Correct: Although inbound rule allows, outbound rule might be too restrictive; security groups are stateful, but if outbound is denied, the response is blocked.
C
The network ACL on the private subnet has an inbound deny rule for port 3306.
Why wrong: Wrong: NACLs are default allow.
D
The VPC is peered with another VPC that has overlapping CIDR.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The database security group does not allow outbound traffic for the response.
Option A is correct because security group rules are stateful; if the security group allows inbound from the app security group, return traffic is automatically allowed. Option B is wrong because VPC peering is not mentioned. Option C is wrong because NACLs are default allow. Option D is wrong because IAM roles are not needed for network connectivity.
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 database instance does not have an IAM role assigned.
Why it's wrong here
Wrong: IAM not needed for connectivity.
✓
The database security group does not allow outbound traffic for the response.
Why this is correct
Correct: Although inbound rule allows, outbound rule might be too restrictive; security groups are stateful, but if outbound is denied, the response is blocked.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The network ACL on the private subnet has an inbound deny rule for port 3306.
Why it's wrong here
Wrong: NACLs are default allow.
✗
The VPC is peered with another VPC that has overlapping CIDR.
Why it's wrong here
Wrong: No peering mentioned.
Common exam traps
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 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.
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 Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The database security group does not allow outbound traffic for the response. — Option A is correct because security group rules are stateful; if the security group allows inbound from the app security group, return traffic is automatically allowed. Option B is wrong because VPC peering is not mentioned. Option C is wrong because NACLs are default allow. Option D is wrong because IAM roles are not needed for network connectivity.
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: "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?
CIDR notation defines the prefix length.
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