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 security engineer is designing a multi-tier web application. The application uses an Application Load Balancer (ALB) to distribute traffic to EC2 instances in private subnets. The engineer needs to ensure that the EC2 instances only accept traffic from the ALB and not from any other source. Which security group configuration should the engineer use?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Allow inbound HTTP traffic from the ALB's security group on the EC2 instances' security group.
Option B is correct because referencing the ALB's security group as the source allows inbound traffic only from the ALB. Option A allows all HTTP traffic. Option C uses the VPC CIDR, which is too broad. Option D uses the ALB's public IP, which is not static and not recommended.
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 HTTP traffic from the ALB's public IP address on the EC2 instances' security group.
Why it's wrong here
The ALB's public IP can change, and this does not account for the ALB's private IP.
✗
Allow inbound HTTP traffic from the VPC CIDR range on the EC2 instances' security group.
Why it's wrong here
This allows traffic from any resource in the VPC, not just the ALB.
✓
Allow inbound HTTP traffic from the ALB's security group on the EC2 instances' security group.
Allow inbound HTTP traffic from 0.0.0.0/0 on the EC2 instances' security group.
Why it's wrong here
This allows traffic from any source, not just the ALB.
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: Allow inbound HTTP traffic from the ALB's security group on the EC2 instances' security group. — Option B is correct because referencing the ALB's security group as the source allows inbound traffic only from the ALB. Option A allows all HTTP traffic. Option C uses the VPC CIDR, which is too broad. Option D uses the ALB's public IP, which is not static and not recommended.
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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