- A
Configure the EC2 instance security group to allow traffic from the ALB's private IP address range.
Why wrong: ALB private IPs can change; using them is not reliable.
- B
Configure the network ACL for the EC2 instance subnet to allow traffic from the ALB security group.
Why wrong: Network ACLs do not support security group IDs as sources.
- C
Configure the EC2 instance security group to allow traffic from the ALB security group.
Referencing the ALB security group as source ensures only ALB traffic reaches instances.
- D
Configure the EC2 instance security group to allow HTTP traffic from 0.0.0.0/0.
Why wrong: This would allow traffic from anywhere, not just the ALB.
Question 1,496 of 1,616
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 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
✓
Configure the EC2 instance security group to allow traffic from the ALB security group.
The correct answer is C. You can reference the security group of the ALB as the source in the EC2 instance security group rule. This ensures that only traffic from the ALB (which uses that security group) is allowed, regardless of the ALB's IP addresses. Option A is wrong because relying on the ALB's private IP address range is not recommended as IP addresses can change when the ALB scales or is replaced. Option B is wrong because network ACLs do not support security group IDs as sources; they operate at the subnet level and only support IP addresses. Option D is wrong because allowing all HTTP traffic from 0.0.0.0/0 would bypass the restriction and allow any source, defeating the purpose.
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.
Related concept
CIDR notation defines the prefix length.
- ✗
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.
Visual reference
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.
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FAQ
Questions learners often ask
What does this DVA-C02 question test?
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. — The correct answer is C. You can reference the security group of the ALB as the source in the EC2 instance security group rule. This ensures that only traffic from the ALB (which uses that security group) is allowed, regardless of the ALB's IP addresses. Option A is wrong because relying on the ALB's private IP address range is not recommended as IP addresses can change when the ALB scales or is replaced. Option B is wrong because network ACLs do not support security group IDs as sources; they operate at the subnet level and only support IP addresses. Option D is wrong because allowing all HTTP traffic from 0.0.0.0/0 would bypass the restriction and allow any source, defeating the purpose.
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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Last reviewed: Jun 20, 2026
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