- A
/20
Why wrong: Incorrect. A /20 subnet provides only 4,096 addresses, which is less than the required ~5,462 per subnet.
- B
/18
Correct. A /18 subnet provides 16,384 addresses, sufficient for the ~5,462 needed, and is the smallest option that meets the requirement.
- C
/22
Why wrong: Incorrect. A /22 subnet provides only 1,024 addresses, far too few.
- D
/21
Why wrong: Incorrect. A /21 subnet provides only 2,048 addresses, also insufficient.
Question 780 of 1,705
Efficient Subnet Allocation with Future Growth Reservations in AWS VPC
This ANS-C01 practice question tests your understanding of network design. 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. A key principle to apply: vPC CIDR. 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 is designing a VPC with a CIDR block of 10.0.0.0/16. The VPC will host multiple environments (dev, test, prod) and requires subnets in three Availability Zones. The network engineer must allocate subnets efficiently while reserving at least 25% of the address space for future growth. What is the minimum subnet size that should be used for each environment?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"least"Why it matters: You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.
Clue:
"minimum / minimize"Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
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
✓
/18
The VPC CIDR is 10.0.0.0/16, providing 65,536 total IP addresses. Reserving 25% for future growth leaves 49,152 addresses for current use across 3 environments (dev, test, prod) and 3 Availability Zones, totaling 9 subnets. Each subnet therefore needs at least 49,152 / 9 ≈ 5,462 addresses. Among the options, /20 (4,096 addresses) is too small, /21 (2,048) and /22 (1,024) are also insufficient. The only option that provides enough addresses is /18 (16,384 addresses), making it the minimum subnet size from the given choices.
Key principle: VPC CIDR
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
/20
Why it's wrong here
Incorrect. A /20 subnet provides only 4,096 addresses, which is less than the required ~5,462 per subnet.
- ✓
/18
Why this is correct
Correct. A /18 subnet provides 16,384 addresses, sufficient for the ~5,462 needed, and is the smallest option that meets the requirement.
Clue confirmation
The clue words "least", "minimum / minimize" in the question point toward this answer.
Related concept
VPC CIDR
- ✗
/22
Why it's wrong here
Incorrect. A /22 subnet provides only 1,024 addresses, far too few.
- ✗
/21
Why it's wrong here
Incorrect. A /21 subnet provides only 2,048 addresses, also insufficient.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Candidates often forget to multiply by 3 Availability Zones, resulting in only 3 subnets. They then calculate 49,152 / 3 = 16,384 addresses per subnet, which incorrectly suggests /18 is too large. Alternatively, they may incorrectly assume /20 (4,096) is enough by dividing 65,536 by 16 (4 environments × 4 AZs) or similar flawed logic.
Detailed technical explanation
How to think about this question
Subnet sizing in VPC design must account for AWS reserved IP addresses (first four and last one in each subnet) and the need for contiguous address blocks for route summarization. The calculation uses the formula: usable addresses = 2^(32 - prefix) - 5, so a /20 yields 4,091 usable addresses, which when multiplied by 9 subnets gives 36,819 addresses, fitting within the 49,152 available after reserving 25% for growth. In practice, network engineers often use a /19 (8,191 usable) to provide headroom for future subnet expansion within each environment, but the minimum to meet the exact requirement is /20.
KKey Concepts to Remember
- VPC CIDR
- Subnet Sizing
- Address Reservation
- Availability Zone Multiplier
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
VPC CIDR
Real-world example
How this comes up in practice
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
Visual reference
What to study next
Got this wrong? Here's your next step.
Review vPC CIDR, then practise related ANS-C01 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Design — This question tests Network Design — VPC CIDR.
What is the correct answer to this question?
The correct answer is: /18 — The VPC CIDR is 10.0.0.0/16, providing 65,536 total IP addresses. Reserving 25% for future growth leaves 49,152 addresses for current use across 3 environments (dev, test, prod) and 3 Availability Zones, totaling 9 subnets. Each subnet therefore needs at least 49,152 / 9 ≈ 5,462 addresses. Among the options, /20 (4,096 addresses) is too small, /21 (2,048) and /22 (1,024) are also insufficient. The only option that provides enough addresses is /18 (16,384 addresses), making it the minimum subnet size from the given choices.
What should I do if I get this ANS-C01 question wrong?
Review vPC CIDR, then practise related ANS-C01 questions on the same topic to reinforce the concept.
Are there clue words in this question I should notice?
Yes — watch for: "least", "minimum / minimize". You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.
What is the key concept behind this question?
VPC CIDR
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Last reviewed: Jul 4, 2026
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