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. 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 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 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
Option C is correct. The security group for the database server allows inbound from the web server's security group, which is correct. However, security groups are stateful, so the return traffic is automatically allowed. The issue is likely that the database server is not listening on port 3306 or the MySQL service is not running. Since ping works, the network layer is fine. Option A is incorrect because NACLs are default allow. Option B is incorrect because the security group rule is correct. Option D is incorrect because the web server is in the same VPC, so it can reach the private IP.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
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 — Option C is correct. The security group for the database server allows inbound from the web server's security group, which is correct. However, security groups are stateful, so the return traffic is automatically allowed. The issue is likely that the database server is not listening on port 3306 or the MySQL service is not running. Since ping works, the network layer is fine. Option A is incorrect because NACLs are default allow. Option B is incorrect because the security group rule is correct. Option D is incorrect because the web server is in the same VPC, so it can reach the private IP.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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