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 company runs a web application on Amazon EC2 instances behind an Application Load Balancer (ALB). The ALB is configured to terminate SSL/TLS and forward traffic to the instances over HTTP. The security team wants to ensure that the instances only accept traffic from the ALB, not from any other source. How can this be achieved?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure the instance security group to allow HTTP traffic only from the ALB's security group.
Option C is correct because using the ALB's security group as the source in the instance security group ensures traffic only from the ALB. Option A is wrong because using the VPC CIDR would allow traffic from any resource in the VPC. Option B is wrong because using the subnet CIDR would allow traffic from any instance in that subnet. Option D is wrong because using a network ACL would affect all instances in the subnet and cannot differentiate by source.
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.
✗
Configure the instance security group to allow HTTP traffic only from the VPC CIDR block.
Why it's wrong here
This would allow traffic from any resource in the VPC, not just the ALB.
✓
Configure the instance security group to allow HTTP traffic only from the ALB's security group.
Why this is correct
Security group referencing ensures traffic only from the ALB.
Configure the network ACL on the instance's subnet to allow HTTP traffic only from the ALB's private IP address.
Why it's wrong here
Network ACLs are stateless and require symmetric rules; also, ALB IPs can change.
✗
Configure the instance security group to allow HTTP traffic only from the subnet CIDR block where the ALB resides.
Why it's wrong here
This would allow traffic from any instance in that subnet, 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 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: Configure the instance security group to allow HTTP traffic only from the ALB's security group. — Option C is correct because using the ALB's security group as the source in the instance security group ensures traffic only from the ALB. Option A is wrong because using the VPC CIDR would allow traffic from any resource in the VPC. Option B is wrong because using the subnet CIDR would allow traffic from any instance in that subnet. Option D is wrong because using a network ACL would affect all instances in the subnet and cannot differentiate by source.
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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