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 runs a multi-tier web application in a VPC with public and private subnets across two Availability Zones. The web tier uses an Application Load Balancer (ALB) in the public subnets, and the application tier uses EC2 instances in private subnets. The database tier uses an RDS MySQL Multi-AZ instance in private subnets. The company has implemented a network ACL (NACL) on the private subnets to allow only traffic from the ALB security group. Recently, the application tier instances are unable to connect to the RDS database. The security group for RDS allows inbound traffic on port 3306 from the application tier security group. The network team has verified that the application tier instances can reach the internet through a NAT Gateway. What is the MOST likely cause of the connectivity issue?
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 NACL on the private subnets is blocking outbound traffic from the application tier to the RDS database.
The NACL on the private subnets is stateful only for inbound rules; outbound traffic is evaluated separately. Since the NACL allows only inbound traffic from the ALB security group but does not have an outbound rule permitting traffic from the application tier to the RDS database (port 3306), the outbound SYN packets from the application instances are dropped, preventing the TCP handshake from completing.
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 on the private subnets is blocking outbound traffic from the application tier to the RDS database.
The RDS security group is not allowing inbound traffic from the application tier security group.
Why it's wrong here
The security group rule is correct.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume NACLs are stateful like security groups, leading them to overlook the need for explicit outbound rules for traffic initiated from within the subnet.
Detailed technical explanation
How to think about this question
NACLs are stateless, meaning you must explicitly allow both inbound and outbound traffic for a bidirectional flow. In this scenario, the application tier initiates a TCP connection to RDS on port 3306; the outbound SYN packet from the application tier must be permitted by the private subnet NACL's outbound rules. Even if inbound rules are correct, missing outbound rules for ephemeral ports (1024-65535) or the specific destination port will silently drop the traffic. This is a common misconfiguration when engineers focus only on inbound security.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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 NACL on the private subnets is blocking outbound traffic from the application tier to the RDS database. — The NACL on the private subnets is stateful only for inbound rules; outbound traffic is evaluated separately. Since the NACL allows only inbound traffic from the ALB security group but does not have an outbound rule permitting traffic from the application tier to the RDS database (port 3306), the outbound SYN packets from the application instances are dropped, preventing the TCP handshake from completing.
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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Question Discussion
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