The answer is that the network ACL on the private subnet is blocking inbound traffic on port 3306. This is the most likely cause because network ACLs are stateless, meaning they evaluate traffic independently in each direction, unlike security groups which are stateful and automatically allow return traffic. Even if the RDS security group explicitly permits inbound traffic from the bastion host’s security group, the NACL acts as an additional layer of defense at the subnet boundary; by default, a custom NACL denies all inbound traffic, so without a specific allow rule for MySQL port 3306, the connection is silently dropped. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the layered security model—specifically how NACLs and security groups interact, and the common trap of assuming a security group rule alone guarantees connectivity. A key memory tip is “NACLs are stateless, security groups are stateful,” so always verify both inbound and outbound rules on the NACL when troubleshooting connectivity between subnets.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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 VPC with multiple subnets. They have an Amazon RDS for MySQL database in a private subnet. The application team needs to access the database for maintenance from a bastion host in a public subnet. The security group for the database allows inbound traffic from the security group of the bastion host on port 3306. However, the application team cannot connect. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The network ACL on the private subnet is blocking inbound traffic on port 3306.
The most likely cause is that the network ACL (NACL) on the private subnet is blocking inbound traffic on port 3306. Even though the security group on the RDS instance allows traffic from the bastion host's security group, NACLs are stateless and must explicitly allow both inbound and outbound traffic. By default, custom NACLs deny all inbound traffic, so unless an inbound rule for port 3306 (MySQL) is added, the connection will be dropped at the subnet boundary.
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 network ACL on the private subnet is blocking inbound traffic on port 3306.
Why this is correct
Network ACLs are stateless and can block traffic even if security groups allow it.
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 database does not have a route to the internet.
Why it's wrong here
Not required for internal connectivity.
✗
The security group rule for the database is incorrectly configured.
Why it's wrong here
It allows inbound from bastion security group.
✗
The bastion host does not have a route to the database subnet.
Why it's wrong here
RDS is in the same VPC, routing should work.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the distinction between stateful security groups and stateless network ACLs, trapping candidates who assume that a security group rule alone is sufficient for connectivity.
Detailed technical explanation
How to think about this question
Network ACLs operate at the subnet level and are stateless, meaning you must explicitly allow both inbound and outbound traffic for ephemeral ports (1024-65535) to support return traffic. In contrast, security groups are stateful and automatically allow return traffic. A common real-world scenario is when a security group allows traffic but the NACL denies it, causing a silent failure that is hard to diagnose without checking both layers of defense.
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 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.
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 network ACL on the private subnet is blocking inbound traffic on port 3306. — The most likely cause is that the network ACL (NACL) on the private subnet is blocking inbound traffic on port 3306. Even though the security group on the RDS instance allows traffic from the bastion host's security group, NACLs are stateless and must explicitly allow both inbound and outbound traffic. By default, custom NACLs deny all inbound traffic, so unless an inbound rule for port 3306 (MySQL) is added, the connection will be dropped at the subnet boundary.
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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