The answer is to allow inbound database traffic from the security group attached to the application instances. This configuration is correct because security group referencing dynamically scales with your Auto Scaling group—when new application instances launch, they inherit the same security group and are automatically granted access without updating any rules. On the AWS Certified Security Specialty SCS-C02 exam, this question tests your understanding of stateful security groups versus stateless network ACLs, and the common trap is choosing the public subnet CIDR, which would permit any instance in that subnet, not just your application tier. Remember that security groups support logical referencing of other groups, making them ideal for dynamic architectures where instance IPs change. A simple memory tip: think “group-to-group, not IP-to-IP” for scalable, secure database access.
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. 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 security engineer is designing a VPC with a public subnet and a private subnet. The private subnet will host a database instance that should only be accessible from the application instances in the public subnet. The application instances use an Auto Scaling group. Which configuration ensures that only the application instances can access the database?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Allow inbound database port from the security group attached to the application instances in the public subnet.
Option B is correct because referencing the application security group as the source allows dynamic scaling. Option A is wrong because referencing the public subnet CIDR would allow any instance in that subnet, not just the application. Option C is wrong because allowing 0.0.0.0/0 would expose the database. Option D is wrong because network ACLs are stateless and cannot reference security groups.
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.
✓
Allow inbound database port from the security group attached to the application instances in the public subnet.
Why this is correct
This dynamically allows traffic from all application instances.
Allow inbound database port from 0.0.0.0/0 in the database security group.
Why it's wrong here
This would expose the database to the internet.
✗
Configure a network ACL on the private subnet to allow the database port from the public subnet CIDR.
Why it's wrong here
Network ACLs are stateless and do not support security group references.
✗
Allow inbound database port from the public subnet CIDR block in the database security group.
Why it's wrong here
This would allow traffic from any instance in the public 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 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: Allow inbound database port from the security group attached to the application instances in the public subnet. — Option B is correct because referencing the application security group as the source allows dynamic scaling. Option A is wrong because referencing the public subnet CIDR would allow any instance in that subnet, not just the application. Option C is wrong because allowing 0.0.0.0/0 would expose the database. Option D is wrong because network ACLs are stateless and cannot reference security groups.
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.
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 security engineer is designing a VPC with public and private subnets. The VPC will host web servers in public subnets and database servers in private subnets. The web servers need to send traffic to the database servers, and the database servers must not have direct internet access. Which TWO configurations should the engineer implement?
medium
A.Use network ACLs to block all inbound traffic to the private subnets.
✓ B.Configure security group rules to allow inbound traffic from the web server security group to the database security group.
✓ C.Do not add a route to an internet gateway in the route table for the private subnets.
D.Attach an internet gateway to the VPC and route the private subnets to it.
E.Add a NAT gateway in the public subnet and route the private subnets to it.
Why B: Option A is correct because security group rules can allow traffic between web and database servers based on the security group ID. Option D is correct because a NAT gateway is not needed for private subnets that don't need internet access; however, the database servers must not have internet access, so a NAT gateway should not be used. Actually, the correct combination is A and C: Use security group rules (A) and ensure no route to an internet gateway for private subnets (C). Option E is also correct? No, E is about NAT gateway which would give internet access. So correct: A and C.
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Question Discussion
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