The correct choice is to create a security group for each tier and configure inbound rules to allow traffic only from the preceding tier’s security group. This design leverages security group chaining, where you reference the security group ID of the web tier in the inbound rule of the application tier, and the application tier’s security group in the database tier’s inbound rule, creating a logical, scalable segmentation without relying on IP addresses. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of stateful security groups versus stateless NACLs, and the common trap is defaulting to CIDR blocks or NACLs, which are less granular and require manual updates. Remember that security group chaining provides dynamic, instance-level control that scales with auto-scaling groups, while NACLs operate at the subnet level and are stateless, requiring separate inbound and outbound rules. A useful memory tip: “Chain the groups, not the IPs” — think of it as a handoff between tiers, where each security group only trusts the one directly behind it.
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 tasked with implementing network segmentation for a multi-tier application. The web tier must be accessible from the internet, but the application tier must only be accessible from the web tier. The database tier must only be accessible from the application tier. All tiers are in the same VPC. Which design meets these requirements?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create a security group for each tier. Configure inbound rules to allow traffic only from the preceding tier's security group.
Option B is correct because security groups can be referenced in inbound rules of other security groups, allowing the web tier SG to allow inbound from the ALB SG, the app tier SG to allow inbound from the web tier SG, and the database tier SG to allow inbound from the app tier SG. Option A is wrong because using CIDR blocks is less granular and does not scale. Option C is wrong because public subnets expose the application tier to the internet. Option D is wrong because NACLs are stateless and require more rules.
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.
✓
Create a security group for each tier. Configure inbound rules to allow traffic only from the preceding tier's security group.
Why this is correct
Security group references provide granular control.
Use a single security group for all instances and use IAM policies to restrict access.
Why it's wrong here
IAM policies do not control network traffic.
✗
Place each tier in separate subnets and use network ACLs with CIDR blocks to allow traffic between tiers.
Why it's wrong here
NACLs are stateless and require complex rules; security groups are preferred.
✗
Place all instances in public subnets and restrict access using security groups.
Why it's wrong here
Public subnets are not recommended for application and database tiers.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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: Create a security group for each tier. Configure inbound rules to allow traffic only from the preceding tier's security group. — Option B is correct because security groups can be referenced in inbound rules of other security groups, allowing the web tier SG to allow inbound from the ALB SG, the app tier SG to allow inbound from the web tier SG, and the database tier SG to allow inbound from the app tier SG. Option A is wrong because using CIDR blocks is less granular and does not scale. Option C is wrong because public subnets expose the application tier to the internet. Option D is wrong because NACLs are stateless and require more rules.
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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