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 is using an Application Load Balancer (ALB) to distribute traffic to a set of EC2 instances in private subnets. The security team wants to ensure that only traffic from the ALB can reach the EC2 instances. Which security group configuration should be applied to the EC2 instances?
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/HTTPS from the security group attached to the ALB.
Option B is correct because referencing the ALB's security group as the source ensures only traffic originating from the ALB can reach the EC2 instances. Option A is wrong because allowing traffic from the VPC CIDR would permit any instance in the VPC to access the EC2 instances, not just the ALB. Option C is wrong because allowing traffic from 0.0.0.0/0 would expose the instances to the internet. Option D is wrong because a network ACL is stateless and does not use security group references.
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/HTTPS from the security group attached to the ALB.
Why this is correct
This ensures only traffic originating from the ALB is allowed.
Configure the network ACL to allow traffic from the ALB's private IP addresses.
Why it's wrong here
Network ACLs are stateless and cannot reference security groups; also IP-based allow is less secure.
✗
Allow inbound HTTP/HTTPS from 0.0.0.0/0.
Why it's wrong here
This would expose the instances to the internet.
✗
Allow inbound HTTP/HTTPS from the VPC CIDR block.
Why it's wrong here
This would allow traffic from any instance in the VPC, 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 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 HTTP/HTTPS from the security group attached to the ALB. — Option B is correct because referencing the ALB's security group as the source ensures only traffic originating from the ALB can reach the EC2 instances. Option A is wrong because allowing traffic from the VPC CIDR would permit any instance in the VPC to access the EC2 instances, not just the ALB. Option C is wrong because allowing traffic from 0.0.0.0/0 would expose the instances to the internet. Option D is wrong because a network ACL is stateless and does not use security group references.
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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