The answer is the custom network ACL associated with the private subnet blocking inbound MySQL traffic from the web server. Security groups are stateful and operate at the instance level, automatically allowing return traffic, but network ACLs are stateless and operate at the subnet level, requiring explicit inbound and outbound rules for both directions. Even if the database server’s security group correctly permits MySQL from the web server’s security group, a restrictive inbound rule in the private subnet’s NACL will drop the traffic before it reaches the instance. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your ability to distinguish between security group and network ACL behavior, a common trap where engineers focus only on instance-level rules while forgetting subnet-level stateless filtering. A key memory tip: security groups are a “smart door” that remembers who knocked, while NACLs are a “bouncer” that checks every single packet both ways—always verify both directions for stateless ACLs.
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. 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 public subnet and a private subnet. The public subnet contains a web server (EC2 instance) that must be accessible from the internet. The private subnet contains a database server (EC2 instance) that should only be accessible from the web server. The web server's security group allows HTTP (80) and HTTPS (443) from 0.0.0.0/0. The database server's security group allows MySQL (3306) from the web server's security group. However, the web server cannot connect to the database server. The network engineer has verified that the web server can reach the internet and that the database server's security group is correctly configured. What is the most likely cause of the connectivity problem?
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 associated with the private subnet is blocking inbound MySQL traffic from the web server.
Option B is correct because the default network ACL in a VPC allows all inbound and outbound traffic, but if a custom NACL is associated with the private subnet, it may block traffic if not properly configured. Option A is wrong because the route table does not affect traffic within the same VPC. Option C is wrong because security groups are stateful, so outbound traffic is automatically allowed. Option D is wrong because internet gateway is not needed for VPC internal traffic.
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 route table for the private subnet does not have a route to the public subnet.
Why it's wrong here
Incorrect: Route tables are not needed for traffic within the same VPC; subnets can communicate by default.
✗
The web server's security group does not allow outbound traffic to the database server.
Why it's wrong here
Incorrect: Security groups are stateful; outbound traffic is allowed if inbound is allowed.
✓
The network ACL associated with the private subnet is blocking inbound MySQL traffic from the web server.
Why this is correct
Correct: A custom NACL can block traffic even if security groups allow it.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The database server does not have a route to the internet gateway.
Why it's wrong here
Incorrect: Internet gateway is not needed for internal VPC communication.
Common exam traps
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 Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The network ACL associated with the private subnet is blocking inbound MySQL traffic from the web server. — Option B is correct because the default network ACL in a VPC allows all inbound and outbound traffic, but if a custom NACL is associated with the private subnet, it may block traffic if not properly configured. Option A is wrong because the route table does not affect traffic within the same VPC. Option C is wrong because security groups are stateful, so outbound traffic is automatically allowed. Option D is wrong because internet gateway is not needed for VPC internal traffic.
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.
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 company has a VPC with a public subnet and a private subnet. The public subnet has a bastion host (EC2) with a security group that allows SSH from a specific IP range. The private subnet has an RDS instance. The company wants to enable the bastion host to connect to the RDS instance. Which TWO steps are required?
medium
A.Configure the RDS instance to use a custom DB parameter group with SSL enabled.
✓ B.Add a rule to the NACL for the private subnet to allow inbound traffic from the bastion host's IP.
C.Assign a public IP address to the RDS instance.
D.Create a route in the public subnet's route table to the RDS instance.
✓ E.Add a rule to the RDS security group that allows inbound traffic from the bastion host's security group.
Why B: The bastion host needs network-level access to the RDS instance. The RDS security group must allow inbound from the bastion's security group, and the bastion must be in the same VPC or have network connectivity. The NACL must allow the traffic.
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Question Discussion
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