The answer is to configure the EC2 instances' security group to allow traffic from the ALB's security group as the source. This works because security groups support referencing other security groups as a source, creating a logical, dynamic trust boundary that automatically scales with the ALB’s elastic network interfaces. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of security group chaining versus stateless network ACLs or IAM-based controls—common traps include suggesting the ALB’s source IP or a client CIDR block, which would allow direct traffic and break the restriction. A key memory tip is to think of the ALB’s security group as a “VIP pass” that only the load balancer can hand out, ensuring no other source can reach the instances.
SCS-C02 Infrastructure Security Practice Question
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 hosts a web application on EC2 instances behind an Application Load Balancer. The security team wants to ensure that only traffic from the ALB can reach the EC2 instances. Which configuration should be applied?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure the instances' security group to allow traffic from the ALB's security group.
Using the ALB's security group as the source in the instance's security group is the best practice. Network ACLs are stateless and less granular, IAM roles do not control network traffic, and source IP from the client would include direct traffic.
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 instances' security group to allow traffic from the ALB's security group.
Why this is correct
This ensures only traffic that passes through the ALB can reach the instances.
Configure the instances' security group to allow traffic from the client's source IP addresses.
Why it's wrong here
This would allow direct traffic bypassing the ALB.
✗
Configure a network ACL on the subnet to allow traffic from the ALB's private IP addresses.
Why it's wrong here
Network ACLs are stateless and would require managing IP changes.
✗
Assign an IAM role to the instances that allows traffic only from the ALB.
Why it's wrong here
IAM roles do not control network traffic.
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: Configure the instances' security group to allow traffic from the ALB's security group. — Using the ALB's security group as the source in the instance's security group is the best practice. Network ACLs are stateless and less granular, IAM roles do not control network traffic, and source IP from the client would include direct traffic.
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 company is running a critical application on EC2 instances behind an Application Load Balancer. The security team wants to ensure that only traffic from the ALB reaches the EC2 instances. How can this be achieved?
medium
A.Use the ALB's private IP address in the EC2 security group.
B.Configure a Network ACL to allow only the ALB's subnet.
✓ C.Reference the ALB's security group in the EC2 security group inbound rule.
D.Use the ALB's public IP address in the EC2 security group.
Why C: Option D is correct because referencing the ALB's security group in the EC2 security group ensures only traffic from the ALB is allowed. Option A is wrong because the ALB's private IP can change. Option B is wrong because public IPs are not used. Option C is wrong because a Network ACL would affect all instances in the subnet.
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This SCS-C02 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SCS-C02 exam.
Question Discussion
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