This SCS-C02 practice question tests your understanding of infrastructure security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 has a requirement to block traffic from specific IP addresses known to be malicious. The company has an Application Load Balancer (ALB) that fronts a web application. The security engineer needs to implement a solution that can block these IP addresses at the edge before they reach the ALB. Which AWS service 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
✓
Use AWS WAF with an IP set rule to block the malicious IP addresses.
Option B is correct. AWS WAF can be attached to an ALB and can block traffic based on IP addresses using IP match conditions. Option A (Security groups) are for instance-level and cannot be attached to ALB. Option C (Network ACLs) are for subnets, not ALB. Option D (AWS Shield Advanced) provides DDoS protection but not IP-based blocking.
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 security groups on the ALB to deny traffic from those IP addresses.
Why it's wrong here
Security groups cannot be attached to ALB; they are for ENIs.
✗
Enable AWS Shield Advanced and configure rate-based rules.
Why it's wrong here
Shield Advanced is for DDoS, not specific IP blocking.
✗
Add a network ACL rule to the ALB's subnet to deny the IP addresses.
Why it's wrong here
NACLs are stateless and not designed for application-level blocking.
✓
Use AWS WAF with an IP set rule to block the malicious IP addresses.
Why this is correct
WAF can block IPs at the edge before reaching the ALB.
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 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: Use AWS WAF with an IP set rule to block the malicious IP addresses. — Option B is correct. AWS WAF can be attached to an ALB and can block traffic based on IP addresses using IP match conditions. Option A (Security groups) are for instance-level and cannot be attached to ALB. Option C (Network ACLs) are for subnets, not ALB. Option D (AWS Shield Advanced) provides DDoS protection but not IP-based blocking.
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.
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Question Discussion
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