← ANS-C01·Amazon Web Services

Question 1,577 of 1,705

Network Design →mediumMultiple ChoiceObjective-mapped

Private Subnet Missing Route: Troubleshooting VPC Connectivity

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 a CIDR block of 10.0.0.0/16. They have two subnets: a public subnet (10.0.1.0/24) and a private subnet (10.0.2.0/24). They launch an Amazon RDS for MySQL DB instance in the private subnet. The DB instance needs to be accessed by an EC2 instance in the public subnet. The security group for the DB instance allows inbound traffic on port 3306 from the security group of the EC2 instance. However, the EC2 instance cannot connect to the DB instance. 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.

A
The private subnet does not have a route to the public subnet
Why wrong: Incorrect. The private subnet's route table automatically includes a local route for the entire VPC CIDR (10.0.0.0/16), which cannot be deleted. Therefore, the private subnet always has a route to the public subnet. A missing route is not a possible cause.
B
The network ACL for the private subnet is blocking inbound traffic
Correct. The network ACL for the private subnet is stateless and must allow inbound traffic on port 3306 from the public subnet. If it has been modified to block this traffic, the connection will fail. By default, NACLs allow all traffic, but custom rules may block it.
C
The security group for the EC2 instance does not allow outbound traffic
Why wrong: Incorrect. Security groups are stateful and allow all outbound traffic by default. Unless the outbound rule has been explicitly removed, the EC2 instance can initiate outbound traffic to the DB instance on port 3306.
D
The DB instance does not have a public IP address
Why wrong: Incorrect. For communication within a VPC, instances communicate using private IP addresses. The DB instance does not need a public IP address because the EC2 instance is in the same VPC.

Full breakdown with real-world context →

Answer choices

Why each option matters

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 for the private subnet is blocking inbound traffic

The most likely cause is that the network ACL (NACL) for the private subnet is blocking inbound traffic on port 3306. In Amazon VPC, each subnet has a default NACL that allows all inbound and outbound traffic by default, but if the NACL has been modified to block inbound traffic on port 3306, the connection from the EC2 instance will be denied. Security groups are stateful and allow return traffic automatically, but NACLs are stateless and require explicit rules for both inbound and outbound traffic. The private subnet's route table automatically includes a local route to the entire VPC CIDR (10.0.0.0/16), which cannot be deleted, so a missing route is not a possible cause. The DB instance does not need a public IP for VPC internal communication, and the EC2 security group likely allows all outbound traffic by default.

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 private subnet does not have a route to the public subnet
Why it's wrong here
Incorrect. The private subnet's route table automatically includes a local route for the entire VPC CIDR (10.0.0.0/16), which cannot be deleted. Therefore, the private subnet always has a route to the public subnet. A missing route is not a possible cause.
✓
The network ACL for the private subnet is blocking inbound traffic
Why this is correct
Correct. The network ACL for the private subnet is stateless and must allow inbound traffic on port 3306 from the public subnet. If it has been modified to block this traffic, the connection will fail. By default, NACLs allow all traffic, but custom rules may block 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 security group for the EC2 instance does not allow outbound traffic
Why it's wrong here
Incorrect. Security groups are stateful and allow all outbound traffic by default. Unless the outbound rule has been explicitly removed, the EC2 instance can initiate outbound traffic to the DB instance on port 3306.
✗
The DB instance does not have a public IP address
Why it's wrong here
Incorrect. For communication within a VPC, instances communicate using private IP addresses. The DB instance does not need a public IP address because the EC2 instance is in the same VPC.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Candidates often assume that security group misconfigurations or missing public IPs are the cause, but they overlook the stateless nature of network ACLs. In a VPC, subnets automatically have a local route, so routing is not an issue. The real trap is failing to check the network ACL for the private subnet when inbound traffic is blocked.

Detailed technical explanation

How to think about this question

In a VPC, subnets are associated with route tables that determine traffic flow. For an EC2 instance in a public subnet (with an internet gateway) to reach a DB instance in a private subnet, the private subnet's route table must have a route pointing to the public subnet's CIDR block (10.0.1.0/24) via a local route or a VPC peering connection. Without this, the DB instance's response packets are dropped because the private subnet has no route back to the public subnet. This is a common misconfiguration in multi-tier architectures where return traffic is overlooked.

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.

Visual reference

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.

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FAQ

Questions learners often ask

What does this ANS-C01 question test?

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 for the private subnet is blocking inbound traffic — The most likely cause is that the network ACL (NACL) for the private subnet is blocking inbound traffic on port 3306. In Amazon VPC, each subnet has a default NACL that allows all inbound and outbound traffic by default, but if the NACL has been modified to block inbound traffic on port 3306, the connection from the EC2 instance will be denied. Security groups are stateful and allow return traffic automatically, but NACLs are stateless and require explicit rules for both inbound and outbound traffic. The private subnet's route table automatically includes a local route to the entire VPC CIDR (10.0.0.0/16), which cannot be deleted, so a missing route is not a possible cause. The DB instance does not need a public IP for VPC internal communication, and the EC2 security group likely allows all outbound traffic by default.

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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