- A
Create an AWS WAF web ACL with an IP set deny rule and associate it with the ALB.
AWS WAF can block requests based on source IP quickly.
- B
Modify the ALB listener rules to drop requests from the client IP.
Why wrong: ALB listener rules do not filter by source IP; they route based on path, host, etc.
- C
Update the ALB security group to add a deny rule for the client IP address.
Why wrong: Security groups do not support deny rules; they only allow.
- D
Update the VPC route table to drop packets from the client IP.
Why wrong: Route tables do not filter traffic by IP; they route based on destination.
Question 982 of 1,750
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 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 deny rule and associate it with the ALB.
The fastest way to block traffic from a specific client IP without affecting other traffic is to use AWS WAF. Option A is correct: Create an AWS WAF web ACL with an IP set deny rule and associate it with the ALB. This immediately blocks the malicious IP at the application layer without altering network infrastructure. Option B is incorrect because ALB listener rules only affect routing decisions (e.g., forwarding to target groups), not packet dropping. Option C is incorrect because security groups only support allow rules; you cannot add a deny rule. You would have to update the security group to remove the allow from 0.0.0.0/0 and add allow for all except that IP, which is not a simple deny and could affect other traffic. Option D is incorrect because VPC route tables operate at the network layer and cannot filter by source IP; they only direct traffic based on destination. Also, modifying route tables would affect all traffic to the ALB's subnets, not just the specific 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.
Related concept
CIDR notation defines the prefix length.
- ✗
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.
Visual reference
Quick reference
OSI Model Reference
| Layer | Name | PDU | Key Protocols / Devices |
|---|---|---|---|
| 7 | Application | Data | HTTP, HTTPS, DNS, SMTP, FTP, SSH |
| 6 | Presentation | Data | TLS / SSL, JPEG, ASCII encoding |
| 5 | Session | Data | NetBIOS, RPC, SIP |
| 4 | Transport | Segment / Datagram | TCP, UDP |
| 3 | Network | Packet | IP, ICMP, OSPF — Routers |
| 2 | Data Link | Frame | Ethernet, Wi-Fi, PPP — Switches, Bridges |
| 1 | Physical | Bits | Cables, NICs, Hubs, Repeaters |
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.
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Incident and Event Response — study guide chapter
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FAQ
Questions learners often ask
What does this DOP-C02 question test?
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. — The fastest way to block traffic from a specific client IP without affecting other traffic is to use AWS WAF. Option A is correct: Create an AWS WAF web ACL with an IP set deny rule and associate it with the ALB. This immediately blocks the malicious IP at the application layer without altering network infrastructure. Option B is incorrect because ALB listener rules only affect routing decisions (e.g., forwarding to target groups), not packet dropping. Option C is incorrect because security groups only support allow rules; you cannot add a deny rule. You would have to update the security group to remove the allow from 0.0.0.0/0 and add allow for all except that IP, which is not a simple deny and could affect other traffic. Option D is incorrect because VPC route tables operate at the network layer and cannot filter by source IP; they only direct traffic based on destination. Also, modifying route tables would affect all traffic to the ALB's subnets, not just the specific 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.
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Last reviewed: Jun 20, 2026
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