- A
Update the security group for the ALB to deny traffic from the IP range.
Why wrong: Security groups are stateful and applied to the ALB, but they work at the instance level; ALB security groups can filter, but WAF is better for edge filtering.
- B
Create an AWS WAF web ACL with an IP set rule that blocks the IP range and attach it to the ALB.
WAF at the ALB blocks traffic before it reaches the backend.
- C
Add a network ACL rule on the ALB's subnet to deny inbound traffic from the IP range.
Why wrong: NACLs are stateless and apply to the subnet, but ALB is in the subnet; however, NACLs are not as granular as WAF and require separate rules for inbound/outbound.
- D
Deploy Amazon CloudFront in front of the ALB and use AWS WAF to block the IP range at CloudFront.
Why wrong: This adds complexity and changes architecture; the question requires edge blocking without changing the existing setup.
Quick Answer
The correct answer is to create an AWS WAF web ACL with an IP set rule that blocks the IP range and attach it to the ALB. This works because AWS WAF operates at the network edge when attached to an Application Load Balancer, inspecting incoming HTTP/HTTPS requests before they reach the ALB itself, making it ideal for blocking specific IP ranges during a DDoS attack. On the ANS-C01 exam, this scenario tests your understanding of where different security controls apply in the AWS shared responsibility model—WAF is the only Layer 7 filtering service that can be attached directly to an ALB to block traffic at the edge, whereas security groups and NACLs operate at the instance or subnet level and cannot filter by IP range before the ALB processes the request. A common trap is confusing NACLs with WAF, but remember that NACLs are stateless and apply at the subnet boundary, meaning traffic still reaches the ALB first. Memory tip: WAF = Web Application Firewall at the edge; think “WAF blocks before ALB talks.”
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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 web application on Amazon EC2 instances behind an Application Load Balancer (ALB). The security team needs to block a specific IP address range that has been launching DDoS attacks. The solution must be implemented at the network edge, before traffic reaches the ALB. What should the security team do?
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
Create an AWS WAF web ACL with an IP set rule that blocks the IP range and attach it to the ALB.
AWS WAF web ACL attached to the ALB can filter traffic based on IP addresses. Option B is correct because WAF is deployed at the edge (ALB or CloudFront) and can block IP sets. Option A is wrong because security groups are stateful but work at the instance level, not at the edge. Option C is wrong because NACLs are applied at the subnet level, but ALB is in a subnet, so traffic reaches the ALB first; NACL is stateless and less suitable for layer 7 filtering. Option D is wrong because CloudFront with WAF is also edge, but the question says 'before traffic reaches the ALB' and ALB is already in place; adding CloudFront changes the architecture.
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.
- ✗
Update the security group for the ALB to deny traffic from the IP range.
Why it's wrong here
Security groups are stateful and applied to the ALB, but they work at the instance level; ALB security groups can filter, but WAF is better for edge filtering.
- ✓
Create an AWS WAF web ACL with an IP set rule that blocks the IP range and attach it to the ALB.
Why this is correct
WAF at the ALB blocks traffic before it reaches the backend.
Related concept
CIDR notation defines the prefix length.
- ✗
Add a network ACL rule on the ALB's subnet to deny inbound traffic from the IP range.
Why it's wrong here
NACLs are stateless and apply to the subnet, but ALB is in the subnet; however, NACLs are not as granular as WAF and require separate rules for inbound/outbound.
- ✗
Deploy Amazon CloudFront in front of the ALB and use AWS WAF to block the IP range at CloudFront.
Why it's wrong here
This adds complexity and changes architecture; the question requires edge blocking without changing the existing setup.
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 ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Create an AWS WAF web ACL with an IP set rule that blocks the IP range and attach it to the ALB. — AWS WAF web ACL attached to the ALB can filter traffic based on IP addresses. Option B is correct because WAF is deployed at the edge (ALB or CloudFront) and can block IP sets. Option A is wrong because security groups are stateful but work at the instance level, not at the edge. Option C is wrong because NACLs are applied at the subnet level, but ALB is in a subnet, so traffic reaches the ALB first; NACL is stateless and less suitable for layer 7 filtering. Option D is wrong because CloudFront with WAF is also edge, but the question says 'before traffic reaches the ALB' and ALB is already in place; adding CloudFront changes the architecture.
What should I do if I get this ANS-C01 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 ANS-C01 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
This ANS-C01 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 ANS-C01 exam.
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