- A
The web server cannot reach the private IP of the database server because it is in a different subnet
Why wrong: Incorrect; within the same VPC, instances can communicate across subnets.
- B
The network ACL for the private subnet is blocking inbound traffic on port 3306
Why wrong: Incorrect; the default NACL allows all inbound traffic.
- C
The security group for the database server is not allowing inbound traffic from the web server's security group
Why wrong: Incorrect; it does allow.
- D
The MySQL service on the database server is not running or not listening on port 3306
Correct; the application layer is the most likely issue.
Question 860 of 1,705
Troubleshooting EC2 Connectivity: Security Groups vs. Service State
This ANS-C01 practice question tests your understanding of network implementation. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: security Group Statefulness. 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 small company has a single VPC with one public subnet and one private subnet. They have a web server in the public subnet and a database server in the private subnet. The web server needs to access the database server on port 3306 (MySQL). The network engineer has configured the security group for the database server to allow inbound TCP port 3306 from the security group of the web server. However, the web server cannot connect to the database server. The network ACL for the private subnet is the default (allows all inbound and outbound). The web server can ping the database server's private IP. What is the MOST likely cause of the connection failure?
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 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 MySQL service on the database server is not running or not listening on port 3306
The database server's security group is correctly configured to allow inbound traffic from the web server's security group on port 3306. The network ACL for the private subnet is default allow, so it is not blocking traffic. The web server can ping the database server's private IP, indicating network connectivity exists. Therefore, the issue is that the MySQL service on the database server is not running or not listening on port 3306. Option D is correct.
Key principle: Security Group Statefulness
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 web server cannot reach the private IP of the database server because it is in a different subnet
Why it's wrong here
Incorrect; within the same VPC, instances can communicate across subnets.
- ✗
The network ACL for the private subnet is blocking inbound traffic on port 3306
Why it's wrong here
Incorrect; the default NACL allows all inbound traffic.
- ✗
The security group for the database server is not allowing inbound traffic from the web server's security group
Why it's wrong here
Incorrect; it does allow.
- ✓
The MySQL service on the database server is not running or not listening on port 3306
Why this is correct
Correct; the application layer is the most likely issue.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Security Group Statefulness
Common exam traps
Common exam trap: answer the scenario, not the keyword
Remember that security groups are stateful and automatically allow return traffic. The default network ACL allows all inbound and outbound traffic. If ping works but a specific port connection fails, the application service may not be running.
Detailed technical explanation
How to think about this question
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- Security Group Statefulness
- Default Network ACL
- Application Service Verification
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
Security Group Statefulness
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.
Review security Group Statefulness, then practise related ANS-C01 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Implementation — This question tests Network Implementation — Security Group Statefulness.
What is the correct answer to this question?
The correct answer is: The MySQL service on the database server is not running or not listening on port 3306 — The database server's security group is correctly configured to allow inbound traffic from the web server's security group on port 3306. The network ACL for the private subnet is default allow, so it is not blocking traffic. The web server can ping the database server's private IP, indicating network connectivity exists. Therefore, the issue is that the MySQL service on the database server is not running or not listening on port 3306. Option D is correct.
What should I do if I get this ANS-C01 question wrong?
Review security Group Statefulness, then practise related ANS-C01 questions on the same topic to reinforce the concept.
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?
Security Group Statefulness
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Last reviewed: Jun 20, 2026
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