This DOP-C02 practice question tests your understanding of security and compliance. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 runs a critical application on EC2 instances behind an Application Load Balancer (ALB). The security team wants to block traffic from known malicious IP addresses before it reaches the ALB. What is the MOST effective approach?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use AWS WAF with an IP set rule to block the malicious IP addresses.
Option D is correct because AWS WAF integrated with ALB can block malicious IPs at the application layer. Option A is wrong because security groups are stateful and can only allow/deny by IP/CIDR, but managing large lists is cumbersome. Option B is wrong because NACLs are stateless and not suitable for fine-grained IP blocking at the application layer. Option C is wrong because Network Firewall is for VPC-level inspection, not specifically for ALB.
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.
✓
Use AWS WAF with an IP set rule to block the malicious IP addresses.
Why this is correct
AWS WAF integrates with ALB and can block IPs via IP sets.
Configure network ACLs on the ALB's subnet to block the malicious IPs.
Why it's wrong here
NACLs are stateless and require careful rule management; not ideal for ALB.
✗
Use AWS Network Firewall to inspect and block traffic at the VPC level.
Why it's wrong here
Network Firewall can block IPs but is more complex and not directly integrated with ALB.
✗
Configure security groups to deny inbound traffic from the malicious IP addresses.
Why it's wrong here
Security groups are stateful but managing large dynamic IP lists is not practical.
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 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 block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DOP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Security and Compliance — This question tests Security and Compliance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Use AWS WAF with an IP set rule to block the malicious IP addresses. — Option D is correct because AWS WAF integrated with ALB can block malicious IPs at the application layer. Option A is wrong because security groups are stateful and can only allow/deny by IP/CIDR, but managing large lists is cumbersome. Option B is wrong because NACLs are stateless and not suitable for fine-grained IP blocking at the application layer. Option C is wrong because Network Firewall is for VPC-level inspection, not specifically for ALB.
What should I do if I get this DOP-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 DOP-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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