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Question 678 of 1,730

Monitoring and Troubleshooting →easyMultiple ChoiceObjective-mapped

DBS-C01 Monitoring and Troubleshooting Practice Question

This DBS-C01 practice question tests your understanding of monitoring and troubleshooting. 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 developer reports that an application is unable to connect to an Amazon RDS for MySQL DB instance. The security group for the DB instance allows inbound traffic on port 3306 from the application server's security group. The DB instance is in a VPC with both public and private subnets. The application server is in a private subnet. 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.

Question 1easymultiple choice

Review the full subnetting walkthrough →

A
The DB instance is in a public subnet and the application server is in a private subnet, so they cannot communicate.
Why wrong: Instances in public and private subnets within the same VPC can communicate via private IPs.
B
The security group for the DB instance does not allow inbound traffic from the application server's security group.
The security group must have an inbound rule allowing traffic from the application server's security group on port 3306.
C
The network ACL for the private subnet is blocking outbound traffic to the DB instance.
Why wrong: Network ACLs are stateless; the outbound rule must allow traffic, but typically it allows all outbound.
D
The DB instance is not part of a DB subnet group that includes the private subnet.
Why wrong: DB subnet groups are for Multi-AZ deployments; a single instance can be in any subnet.

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 security group for the DB instance does not allow inbound traffic from the application server's security group.

If the DB instance is in a public subnet and the application server is in a private subnet, they are in different subnets within the same VPC, so they can communicate via private IPs if routing is correct. However, the most common issue is that the security group rule is not correctly associated or the DB instance is in a public subnet with a public DNS name that resolves to a public IP, but the private subnet cannot reach the internet. But the question implies the DB instance is in a public subnet; private subnet instances can reach public subnet instances via the VPC router. However, often the misconfiguration is that the DB instance's security group does not allow traffic from the application server's security group. Option D is correct because the security group rule might be missing or incorrect. Option A is wrong because NACLs are stateless and typically allow all outbound. Option B is wrong because the DB instance is in a public subnet, but the application server is in a private subnet, they can communicate within the VPC. Option C is wrong because the DB subnet group is for Multi-AZ, not single instance.

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 DB instance is in a public subnet and the application server is in a private subnet, so they cannot communicate.
Why it's wrong here
Instances in public and private subnets within the same VPC can communicate via private IPs.
✓
The security group for the DB instance does not allow inbound traffic from the application server's security group.
Why this is correct
The security group must have an inbound rule allowing traffic from the application server's security group on port 3306.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The network ACL for the private subnet is blocking outbound traffic to the DB instance.
Why it's wrong here
Network ACLs are stateless; the outbound rule must allow traffic, but typically it allows all outbound.
✗
The DB instance is not part of a DB subnet group that includes the private subnet.
Why it's wrong here
DB subnet groups are for Multi-AZ deployments; a single instance can be in any subnet.

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 DBS-C01 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this DBS-C01 question test?

Monitoring and Troubleshooting — This question tests Monitoring and Troubleshooting — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The security group for the DB instance does not allow inbound traffic from the application server's security group. — If the DB instance is in a public subnet and the application server is in a private subnet, they are in different subnets within the same VPC, so they can communicate via private IPs if routing is correct. However, the most common issue is that the security group rule is not correctly associated or the DB instance is in a public subnet with a public DNS name that resolves to a public IP, but the private subnet cannot reach the internet. But the question implies the DB instance is in a public subnet; private subnet instances can reach public subnet instances via the VPC router. However, often the misconfiguration is that the DB instance's security group does not allow traffic from the application server's security group. Option D is correct because the security group rule might be missing or incorrect. Option A is wrong because NACLs are stateless and typically allow all outbound. Option B is wrong because the DB instance is in a public subnet, but the application server is in a private subnet, they can communicate within the VPC. Option C is wrong because the DB subnet group is for Multi-AZ, not single instance.

What should I do if I get this DBS-C01 question wrong?

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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