The answer is the network ACL for the database subnet blocking inbound traffic on port 3306. This is correct because network ACLs are stateless and apply to the entire subnet, meaning they act as a subnet-level firewall that must explicitly allow traffic, regardless of the security group rules on the database instance. While the database’s security group permits inbound traffic from the application’s security group, the NACL is evaluated first at the subnet boundary, and custom NACLs default to denying all inbound traffic. On the CompTIA Cloud+ CV0-004 exam, this scenario tests your understanding of the layered defense model in a VPC, where security groups are stateful instance-level filters and NACLs are stateless subnet-level filters—a common trap is assuming security group rules alone guarantee connectivity. Remember the mnemonic: “NACL blocks the subnet, SG lets the instance in.”
CV0-004 Operations and Support Practice Question
This CV0-004 practice question tests your understanding of operations and support. 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 cloud administrator is troubleshooting an application that fails to connect to a database. The database server is in a private subnet, and the application server is in a public subnet. The security group for the database allows inbound traffic on port 3306 from the application's security group. Which of the following is the MOST likely reason the connection fails?
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 for the database subnet is blocking inbound traffic on port 3306
The network ACL (NACL) for the database subnet is the most likely cause because NACLs are stateless and apply to the entire subnet. Even if the database security group allows inbound traffic from the application's security group, the NACL must also allow inbound traffic on port 3306 (MySQL) from the application subnet. By default, custom NACLs deny all inbound traffic, so if the administrator did not explicitly add a rule for port 3306, the connection will be blocked 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 for the database subnet is blocking inbound traffic on port 3306
Why this is correct
NACLs are stateless and require explicit allow rules.
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 server's operating system firewall is blocking the connection
Why it's wrong here
Could be, but security group is stateful and properly configured; OS firewall is less common in cloud.
✗
The application's security group does not allow outbound traffic to the database
Why it's wrong here
Outbound is usually allowed by default.
✗
The route table for the public subnet does not have a route to the private subnet
Why it's wrong here
Within a VPC, subnets can communicate by default.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the difference between stateful security groups and stateless network ACLs, and the trap here is that candidates assume security group rules are sufficient for connectivity, forgetting that NACLs must also permit the traffic at the subnet boundary.
Detailed technical explanation
How to think about this question
Network ACLs operate at the subnet level and are evaluated before security groups. They are stateless, meaning you must explicitly allow both inbound and outbound traffic separately. For a database in a private subnet, the NACL must have an inbound rule allowing port 3306 from the application subnet's CIDR and an outbound rule allowing ephemeral ports (1024-65535) for the response traffic. In contrast, security groups are stateful, so only inbound rules need to be configured; outbound responses are automatically permitted. This distinction is critical in multi-tier architectures where NACLs are often used as a second layer 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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
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.
Operations and Support — This question tests Operations and Support — 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 database subnet is blocking inbound traffic on port 3306 — The network ACL (NACL) for the database subnet is the most likely cause because NACLs are stateless and apply to the entire subnet. Even if the database security group allows inbound traffic from the application's security group, the NACL must also allow inbound traffic on port 3306 (MySQL) from the application subnet. By default, custom NACLs deny all inbound traffic, so if the administrator did not explicitly add a rule for port 3306, the connection will be blocked at the subnet boundary.
What should I do if I get this CV0-004 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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A cloud administrator is troubleshooting an application that fails to connect to a database. The application and database are in the same VPC. Which THREE steps should the administrator take to diagnose the issue?
hard
A.Check the routing table for a route to the internet.
✓ B.Test connectivity to the database using a telnet or netcat command from the application server.
✓ C.Verify that the security group associated with the database instance allows inbound traffic from the application's security group on the database port.
D.Check the DNS resolution of the database endpoint in the application's subnet.
✓ E.Verify that the network ACL for the database subnet allows inbound traffic on the database port.
Why B: Option B is correct because telnet or netcat can test basic TCP connectivity to the database port, confirming whether the database is reachable from the application server at the network layer. This step isolates whether the issue is a network connectivity problem versus an authentication or configuration issue within the database itself.
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Question Discussion
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