The answer is to create an AWS WAF web ACL with an IP set deny rule and associate it with the ALB. This is the fastest way to block a specific IP address at the Application Load Balancer because AWS WAF operates at the application layer, inspecting incoming HTTP/HTTPS requests and allowing you to deny traffic from a single IP without modifying any network infrastructure. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of the critical difference between security groups, which are stateful and only support allow rules, and WAF, which is designed for granular request filtering. A common trap is assuming you can add a deny rule to a security group, but that is impossible; instead, you must use WAF or a network ACL, though WAF is faster here because it avoids subnet-level changes. Memory tip: “Security groups say yes, WAF says no”—remember that security groups only allow, while WAF can explicitly deny individual IPs.
DOP-C02 Incident and Event Response Practice Question
This DOP-C02 practice question tests your understanding of incident and event response. 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 uses an Application Load Balancer (ALB) in front of a fleet of EC2 instances. The security team reports that a specific client IP address is sending malicious requests and must be blocked immediately. The ALB's security group only allows HTTP/HTTPS from 0.0.0.0/0. What is the FASTEST way to block traffic from this IP address without affecting other traffic?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "immediately / without restart"
Why it matters: Time or reboot constraint — the correct answer must take effect right away without requiring a reboot or reload.
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 deny rule and associate it with the ALB.
Option C is correct because updating the ALB's security group to deny the specific IP address would block all traffic from that IP inbound to the ALB, but the current security group allows all traffic, so you would need to modify it to deny that IP. However, security groups are stateful and cannot deny rules; they only allow. The correct approach is to use a network ACL on the ALB's subnets, which is not listed. Among the options, Option C (creating a WAF web ACL and associating it with the ALB) is the fastest and most appropriate because AWS WAF can block requests based on IP addresses immediately without changing network infrastructure. Option A is wrong because updating the security group to deny is not possible; security groups only support allow rules. Option B is wrong because modifying the ALB's listener rules does not block traffic; it only affects routing. Option D is wrong because updating route tables would affect all traffic to the ALB, not just that IP.
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.
✓
Create an AWS WAF web ACL with an IP set deny rule and associate it with the ALB.
Why this is correct
AWS WAF can block requests based on source IP quickly.
Clue confirmation
The clue word "immediately / without restart" in the question point toward this answer.
Modify the ALB listener rules to drop requests from the client IP.
Why it's wrong here
ALB listener rules do not filter by source IP; they route based on path, host, etc.
✗
Update the ALB security group to add a deny rule for the client IP address.
Why it's wrong here
Security groups do not support deny rules; they only allow.
✗
Update the VPC route table to drop packets from the client IP.
Why it's wrong here
Route tables do not filter traffic by IP; they route based on destination.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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: Create an AWS WAF web ACL with an IP set deny rule and associate it with the ALB. — Option C is correct because updating the ALB's security group to deny the specific IP address would block all traffic from that IP inbound to the ALB, but the current security group allows all traffic, so you would need to modify it to deny that IP. However, security groups are stateful and cannot deny rules; they only allow. The correct approach is to use a network ACL on the ALB's subnets, which is not listed. Among the options, Option C (creating a WAF web ACL and associating it with the ALB) is the fastest and most appropriate because AWS WAF can block requests based on IP addresses immediately without changing network infrastructure. Option A is wrong because updating the security group to deny is not possible; security groups only support allow rules. Option B is wrong because modifying the ALB's listener rules does not block traffic; it only affects routing. Option D is wrong because updating route tables would affect all traffic to the ALB, not just that IP.
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.
Are there clue words in this question I should notice?
Yes — watch for: "immediately / without restart". Time or reboot constraint — the correct answer must take effect right away without requiring a reboot or reload.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company runs a web application on EC2 instances behind an ALB. The security team notices that the ALB is receiving a large number of requests from a single IP address, causing high CPU on the instances. They want to block this IP at the load balancer level without affecting other traffic. The ALB currently has a default action of forwarding to the target group. What is the MOST effective way to block this IP?
hard
A.Update the network ACL for the VPC subnets to deny inbound traffic from that IP.
B.Add a deny rule in the ALB's security group for the source IP.
C.Change the listener rule to forward requests from that IP to a different target group with no instances.
✓ D.Create an AWS WAF web ACL with an IP match condition and associate it with the ALB.
Why D: Option B is correct because AWS WAF can be integrated with ALB to create a rule that blocks requests from a specific IP address. Option A is wrong because security groups cannot deny traffic; they only allow. Option C is wrong because modifying the listener rule's target group would not block the IP; it would just route elsewhere. Option D is wrong because the network ACL is for subnets, not for the ALB directly, and would affect all traffic to the subnets.
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Question Discussion
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