- A
Keep the existing rule that allows inbound HTTP from 0.0.0.0/0, but add a network ACL to block traffic from the internet.
Why wrong: Network ACLs are stateless and would block return traffic; also, the security group should be restrictive.
- B
Modify the EC2 security group to allow inbound HTTP from the ALB's security group.
This ensures only traffic that has passed through the ALB can reach the instances.
- C
Modify the EC2 security group to allow inbound HTTP from the ALB's private IP addresses.
Why wrong: The ALB's private IPs can change; using the security group ID is more dynamic and scalable.
- D
Modify the EC2 security group to allow inbound HTTP from the ALB's public IP addresses.
Why wrong: ALBs do not have static public IPs; they are accessed via DNS name.
Quick Answer
The correct configuration is to modify the EC2 security group to allow inbound HTTP traffic from the ALB’s security group, rather than from an IP range. This approach, known as security group referencing, ties the inbound rule to the ALB’s security group ID, so any EC2 instance associated with that ALB automatically receives traffic without needing to track changing IP addresses. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of stateful security groups versus stateless network ACLs, and it frequently appears as a trap where candidates mistakenly use the ALB’s private or public IPs—both of which are dynamic and unreliable. The core principle is that security group referencing decouples traffic rules from IP management, ensuring scalability and resilience. A helpful memory tip: “Reference the group, not the IP—keep your rules dynamic, not static.”
SOA-C02 Networking and Content Delivery Practice Question
This SOA-C02 practice question tests your understanding of networking and content delivery. 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 deploys a web application on EC2 instances behind an Application Load Balancer. The SysOps administrator needs to allow inbound traffic only from the ALB to the EC2 instances. Currently, the EC2 security group allows inbound HTTP from 0.0.0.0/0. Which security group configuration should the administrator apply?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Modify the EC2 security group to allow inbound HTTP from the ALB's security group.
The best practice is to reference the ALB's security group ID in the EC2 security group's inbound rule, allowing traffic from that security group. Option A is incorrect because 0.0.0.0/0 allows all traffic, defeating the purpose. Option C is incorrect because the ALB's private IPs can change. Option D is incorrect because the ALB's public IPs are not fixed and should not be used.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Keep the existing rule that allows inbound HTTP from 0.0.0.0/0, but add a network ACL to block traffic from the internet.
Why it's wrong here
Network ACLs are stateless and would block return traffic; also, the security group should be restrictive.
- ✓
Modify the EC2 security group to allow inbound HTTP from the ALB's security group.
Why this is correct
This ensures only traffic that has passed through the ALB can reach the instances.
Related concept
Standard ACLs match source addresses.
- ✗
Modify the EC2 security group to allow inbound HTTP from the ALB's private IP addresses.
Why it's wrong here
The ALB's private IPs can change; using the security group ID is more dynamic and scalable.
- ✗
Modify the EC2 security group to allow inbound HTTP from the ALB's public IP addresses.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
- Standard ACLs match source addresses.
- Extended ACLs can match source, destination, protocol and ports.
- The first matching ACL entry is used.
- There is usually an implicit deny at the end.
TExam Day Tips
- Check inbound versus outbound direction.
- Read the ACL from top to bottom.
- Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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 ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related SOA-C02 ACL questions on filtering logic and placement.
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Networking and Content Delivery — study guide chapter
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Networking and Content Delivery practice questions
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FAQ
Questions learners often ask
What does this SOA-C02 question test?
Networking and Content Delivery — This question tests Networking and Content Delivery — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: Modify the EC2 security group to allow inbound HTTP from the ALB's security group. — The best practice is to reference the ALB's security group ID in the EC2 security group's inbound rule, allowing traffic from that security group. Option A is incorrect because 0.0.0.0/0 allows all traffic, defeating the purpose. Option C is incorrect because the ALB's private IPs can change. Option D is incorrect because the ALB's public IPs are not fixed and should not be used.
What should I do if I get this SOA-C02 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related SOA-C02 ACL questions on filtering logic and placement.
What is the key concept behind this question?
Standard ACLs match source addresses.
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Last reviewed: Jun 20, 2026
This SOA-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 SOA-C02 exam.
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