The answer is that the security group for the DB instance does not allow inbound traffic from the EC2 instance. This is the most likely cause when an RDS DB instance is not accessible from an EC2 instance in the same VPC because security groups act as a virtual firewall, and by default, all inbound traffic is denied. Even when both resources share the same VPC and subnets, the DB instance’s security group must explicitly include an inbound rule that references the EC2 instance’s security group as the source, typically on port 3306 for MySQL or the appropriate database port. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of network-level security controls versus routing or public IP configurations—a common trap is to assume the issue lies with subnet routing or internet gateways, but internal VPC communication relies solely on security group rules. Remember the memory tip: “Security groups are stateful and deny by default; if your EC2 can’t reach RDS, check the inbound rules first.”
SOA-C02 Deployment, Provisioning, and Automation Practice Question
This SOA-C02 practice question tests your understanding of deployment, provisioning, and automation. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 uses AWS CloudFormation to manage infrastructure. They have a stack that creates an Amazon RDS DB instance. The database is in a VPC with public and private subnets. The DB instance is in a private subnet. When the stack is created, the DB instance is not accessible from an EC2 instance in the same VPC. What is the most likely cause?
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 security group for the DB instance does not allow inbound traffic from the EC2 instance.
Option D is correct because the DB instance's security group must allow inbound traffic from the EC2 instance's security group. Option A is wrong because the DB instance is in a private subnet, so it doesn't have a public IP. Option B is wrong because the DB subnet group is configured correctly. Option C is wrong because the VPC has a route to the internet, but that's not needed for internal communication.
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 subnet group does not include the correct subnets.
Why it's wrong here
If the subnet group were incorrect, the DB instance would fail to launch.
✓
The security group for the DB instance does not allow inbound traffic from the EC2 instance.
Why this is correct
Security groups control inbound traffic; the DB instance's security group must allow traffic from the EC2 security group.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The VPC does not have an internet gateway attached.
Why it's wrong here
Internet gateway is not required for private subnet communication.
✗
The DB instance does not have a public IP address.
Why it's wrong here
The DB instance is in a private subnet and should not have a public IP; access should be via private IP.
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 SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Deployment, Provisioning, and Automation — This question tests Deployment, Provisioning, and Automation — 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 EC2 instance. — Option D is correct because the DB instance's security group must allow inbound traffic from the EC2 instance's security group. Option A is wrong because the DB instance is in a private subnet, so it doesn't have a public IP. Option B is wrong because the DB subnet group is configured correctly. Option C is wrong because the VPC has a route to the internet, but that's not needed for internal communication.
What should I do if I get this SOA-C02 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 SOA-C02 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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