The correct answer is to configure the EC2 instance security group to allow traffic from the ALB security group. This works because security groups support referencing other security groups as a source, creating a logical dependency that permits only traffic originating from the associated ALB, regardless of the ALB’s underlying IP addresses. On the AWS Certified Developer Associate DVA-C02 exam, this scenario tests your understanding of security group chaining versus IP-based rules, a common trap where candidates mistakenly use the ALB’s private IPs—which can change during scaling events—or suggest network ACLs, which cannot reference security group IDs. A key memory tip is “group-to-group, not IP-to-group”: always link security groups by ID to maintain dynamic, scalable access control.
DVA-C02 Security Practice Question
This DVA-C02 practice question tests your understanding of 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 route traffic to a set of EC2 instances. The security team wants to ensure that only traffic from the ALB can reach the instances. Which security group configuration should be used?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure the EC2 instance security group to allow traffic from the ALB security group.
Option A is correct because referencing the ALB security group as the source in the EC2 instance security group ensures that only traffic originating from the ALB is allowed. Option B is wrong because the ALB's private IP addresses can change if the ALB scales. Option C is wrong because a network ACL does not support security group IDs as sources. Option D is wrong because allowing all HTTP traffic would defeat the purpose of restricting traffic to the ALB only.
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 EC2 instance security group to allow traffic from the ALB's private IP address range.
Why it's wrong here
ALB private IPs can change; using them is not reliable.
✗
Configure the network ACL for the EC2 instance subnet to allow traffic from the ALB security group.
Why it's wrong here
Network ACLs do not support security group IDs as sources.
✓
Configure the EC2 instance security group to allow traffic from the ALB security group.
Why this is correct
Referencing the ALB security group as source ensures only ALB traffic reaches instances.
Configure the EC2 instance security group to allow HTTP traffic from 0.0.0.0/0.
Why it's wrong here
This would allow traffic from anywhere, 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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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 DVA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Security — This question tests Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Configure the EC2 instance security group to allow traffic from the ALB security group. — Option A is correct because referencing the ALB security group as the source in the EC2 instance security group ensures that only traffic originating from the ALB is allowed. Option B is wrong because the ALB's private IP addresses can change if the ALB scales. Option C is wrong because a network ACL does not support security group IDs as sources. Option D is wrong because allowing all HTTP traffic would defeat the purpose of restricting traffic to the ALB only.
What should I do if I get this DVA-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 DVA-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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