- A
Suspend the auto scaling group to prevent additional instances from being launched.
Why wrong: Suspending the group does not stop the current instance from communicating; the malicious traffic continues.
- B
Remove the instance from the auto scaling group and attach a security group that blocks all traffic.
This isolates the specific instance without affecting other instances, and blocking traffic stops the malicious communication.
- C
Use a routing table blackhole to drop traffic from the instance.
Why wrong: Blackholing traffic via routing is complex and could be disruptive; also, routing tables affect all traffic in the VPC.
- D
Modify the network ACL for the subnet to deny traffic to the malicious IP.
Why wrong: Network ACL changes affect all instances in the subnet, potentially causing collateral damage to other instances.
CV0-004 Security Practice Question
This CV0-004 practice question tests your understanding of security. 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 has deployed a multi-tier application on a public cloud platform. The security team discovers that a Compute Instance is communicating with an external IP address known for malicious activity. The instance is part of an auto scaling group. What is the BEST immediate action to contain the threat while minimizing downtime?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"best"Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
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
Remove the instance from the auto scaling group and attach a security group that blocks all traffic.
Isolating the compromised instance by detaching it from the auto scaling group and applying a security group that denies all traffic can quickly contain the threat. Network ACLs are attached to subnets and changing them could affect other instances. Suspending the auto scaling group alone may not stop the current instance. Updating routing tables is complex and could disrupt legitimate traffic.
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.
- ✗
Suspend the auto scaling group to prevent additional instances from being launched.
Why it's wrong here
Suspending the group does not stop the current instance from communicating; the malicious traffic continues.
- ✓
Remove the instance from the auto scaling group and attach a security group that blocks all traffic.
Why this is correct
This isolates the specific instance without affecting other instances, and blocking traffic stops the malicious communication.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
- ✗
Use a routing table blackhole to drop traffic from the instance.
Why it's wrong here
Blackholing traffic via routing is complex and could be disruptive; also, routing tables affect all traffic in the VPC.
- ✗
Modify the network ACL for the subnet to deny traffic to the malicious IP.
Why it's wrong here
Network ACL changes affect all instances in the subnet, potentially causing collateral damage to other instances.
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement 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 CV0-004 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this CV0-004 question test?
Security — This question tests Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Remove the instance from the auto scaling group and attach a security group that blocks all traffic. — Isolating the compromised instance by detaching it from the auto scaling group and applying a security group that denies all traffic can quickly contain the threat. Network ACLs are attached to subnets and changing them could affect other instances. Suspending the auto scaling group alone may not stop the current instance. Updating routing tables is complex and could disrupt legitimate traffic.
What should I do if I get this CV0-004 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 CV0-004 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Last reviewed: Jun 24, 2026
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