This SCS-C02 practice question tests your understanding of infrastructure security. 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 has a VPC with a public subnet and a private subnet. An Amazon RDS instance is in the private subnet, and an application server is in the public subnet. The security team needs to allow the application server to connect to the RDS instance on port 3306 (MySQL). Which configuration will meet this requirement securely?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add an inbound rule to the RDS security group that allows traffic from the security group of the application server on port 3306.
Security groups are stateful; you can allow inbound traffic from the security group of the application server. Option A is correct. Option B is less secure because it allows all traffic from the subnet. Option C is incorrect because it allows all traffic from the VPC. Option D is incorrect because it allows traffic from the internet.
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.
✗
Add an inbound rule to the RDS security group that allows traffic from the VPC CIDR on port 3306.
Why it's wrong here
This allows traffic from all instances in the VPC, which is too broad.
✓
Add an inbound rule to the RDS security group that allows traffic from the security group of the application server on port 3306.
Why this is correct
This restricts access to only the application server's security group.
Add an inbound rule to the RDS security group that allows traffic from the subnet CIDR of the application server on port 3306.
Why it's wrong here
This allows any instance in that subnet, which may include other instances.
✗
Add an inbound rule to the RDS security group that allows traffic from 0.0.0.0/0 on port 3306.
Why it's wrong here
This exposes the database to the internet.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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 SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Infrastructure Security — This question tests Infrastructure Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Add an inbound rule to the RDS security group that allows traffic from the security group of the application server on port 3306. — Security groups are stateful; you can allow inbound traffic from the security group of the application server. Option A is correct. Option B is less secure because it allows all traffic from the subnet. Option C is incorrect because it allows all traffic from the VPC. Option D is incorrect because it allows traffic from the internet.
What should I do if I get this SCS-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 SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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