The correct answer is to update the ALB security group to deny inbound traffic from the IP address and to use AWS WAF to block the IP based on a rule. Security groups act as a virtual firewall for your Application Load Balancer, allowing you to explicitly deny traffic from a specific source IP at the network level, which is immediate and effective. AWS WAF provides a more granular, application-layer control by creating an IP set rule that blocks the malicious address, integrating directly with the ALB to inspect and filter HTTP/HTTPS requests. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your ability to differentiate between network-layer and application-layer mitigation tools—a common trap is confusing security groups with NACLs, which operate at the subnet level and cannot be attached directly to an ALB. Remember, for blocking a specific IP at the load balancer, think "SG for quick network deny, WAF for smart app-layer block."
DOP-C02 Incident and Event Response Practice Question
This DOP-C02 practice question tests your understanding of incident and event response. 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.
During an incident, a DevOps engineer needs to block traffic from a specific IP address that is attacking an Application Load Balancer. Which TWO actions can the engineer take to mitigate this?
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 to create an IP set rule that blocks the IP address.
Option B is correct because updating the security group of the ALB to deny the IP is effective. Option D is correct because AWS WAF can block IPs based on a rule. Option A is wrong because NACLs are for subnets, not ALBs directly. Option C is wrong because CloudFront is a CDN layer, not directly applicable. Option E is wrong because VPC Flow Logs are for monitoring, not 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 Amazon CloudFront to block the IP address.
Why it's wrong here
CloudFront can block IPs, but it's an additional layer not directly on the ALB.
✗
Modify the network ACL for the ALB's subnet to deny the IP address.
Why it's wrong here
NACLs apply at subnet level, but ALBs operate at layer 7; NACL may not block if traffic goes through other paths.
✓
Use AWS WAF to create an IP set rule that blocks the IP address.
Why this is correct
AWS WAF integrates with ALB to filter requests based on IP.
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.
Incident and Event Response — This question tests Incident and Event Response — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Use AWS WAF to create an IP set rule that blocks the IP address. — Option B is correct because updating the security group of the ALB to deny the IP is effective. Option D is correct because AWS WAF can block IPs based on a rule. Option A is wrong because NACLs are for subnets, not ALBs directly. Option C is wrong because CloudFront is a CDN layer, not directly applicable. Option E is wrong because VPC Flow Logs are for monitoring, not blocking.
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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