- A
10.0.0.0/24
Why wrong: /24 gives 256 addresses, insufficient.
- B
10.0.0.0/20
/20 gives 4096 addresses, sufficient and smallest.
- C
10.0.0.0/22
Why wrong: /22 gives 1024 addresses, insufficient.
- D
10.0.0.0/19
Why wrong: /19 gives 8192 addresses, larger than needed.
Question 617 of 1,748
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure 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 a VPC with an IPv4 CIDR block of 10.0.0.0/16. The security team wants to create a subnet for a legacy application that requires 2000 IP addresses. What is the smallest subnet CIDR that meets this requirement?
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
✓
10.0.0.0/20
The /20 subnet provides 4096 total IP addresses (2^(32-20) = 4096), minus 5 reserved by AWS (network, VPC router, DNS, future, broadcast) leaves 4091 usable IPs, which exceeds the 2000 requirement. A /22 offers only 1024 usable IPs (1024 total - 5 = 1019), which is insufficient. The smallest subnet that meets the 2000-IP requirement is /20.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
10.0.0.0/24
Why it's wrong here
/24 gives 256 addresses, insufficient.
- ✓
10.0.0.0/20
Why this is correct
/20 gives 4096 addresses, sufficient and smallest.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
10.0.0.0/22
Why it's wrong here
/22 gives 1024 addresses, insufficient.
- ✗
10.0.0.0/19
Why it's wrong here
/19 gives 8192 addresses, larger than needed.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often forget the 5 AWS-reserved IPs per subnet and calculate usable addresses as 2^(32-subnet_mask) - 2 (as in traditional networking), leading them to incorrectly select /22 (which would seem to offer 1022 usable IPs, but actually only offers 1019).
Detailed technical explanation
How to think about this question
AWS reserves 5 IP addresses per subnet: the network address (first), VPC router (second), DNS (third), future use (second-to-last), and broadcast (last). The formula for usable IPs is 2^(32-subnet_mask) - 5. For /20: 4096 - 5 = 4091 usable; for /22: 1024 - 5 = 1019 usable. This reservation is defined in the AWS VPC documentation and differs from standard RFC 1918 subnet calculations.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
Visual reference
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this SCS-C02 question test?
Infrastructure Security — This question tests Infrastructure Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 10.0.0.0/20 — The /20 subnet provides 4096 total IP addresses (2^(32-20) = 4096), minus 5 reserved by AWS (network, VPC router, DNS, future, broadcast) leaves 4091 usable IPs, which exceeds the 2000 requirement. A /22 offers only 1024 usable IPs (1024 total - 5 = 1019), which is insufficient. The smallest subnet that meets the 2000-IP requirement is /20.
What should I do if I get this SCS-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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