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. 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 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.
/20
Provides 4094 addresses per subnet, fits within the reserved space and allows growth.
/18
Why wrong: Provides 16382 addresses per subnet, too large for the available space.
/22
Why wrong: Provides 1022 addresses per subnet, insufficient for production environments.
/21
Why wrong: Provides 2046 addresses per subnet, may be too small for some environments.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
/20
Option B is correct. The total /16 has 65536 addresses. Reserving 25% leaves 49152 addresses for use. With 3 environments × 3 AZs = 9 subnets, each subnet gets 5461 addresses on average. A /19 subnet provides 8191 addresses per subnet, totaling 9×8191=73719, which exceeds the available 49152. A /20 provides 4095 addresses per subnet (9×4095=36855), which is within the available 49152 and allows efficient use. The smallest subnet that fits the requirement is /20. Option A (/18) is too large, C (/21) provides too few addresses per subnet, D (/22) also insufficient.
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
For each option: why learners choose it and why it is or isn't the right answer here.
/20
Why this is correct
Provides 4094 addresses per subnet, fits within the reserved space and allows growth.
Clue confirmation
The clue words "least", "minimum / minimize" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
/18
Why it's wrong here
Provides 16382 addresses per subnet, too large for the available space.
/22
Why it's wrong here
Provides 1022 addresses per subnet, insufficient for production environments.
/21
Why it's wrong here
Provides 2046 addresses per subnet, may be too small for some environments.
Common exam traps
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
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.
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
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.
What to study next
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Design — study guide chapter
Learn the concepts, then practise the questions
Network Design practice questions
Targeted practice on this topic area only
All ANS-C01 questions
1,705 questions across all exam domains
AWS Certified Advanced Networking Specialty ANS-C01 study guide
Full concept coverage aligned to exam objectives
ANS-C01 practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
Practise ANS-C01 questions linked to Network Management and Operations.
Practise ANS-C01 questions linked to Network Security, Compliance and Governance.
Practise ANS-C01 questions linked to Network Design.
Practise ANS-C01 questions linked to Network Implementation.
Practise ANS-C01 questions linked to ANS-C01 fundamentals.
Practise ANS-C01 questions linked to ANS-C01 scenario.
Practise ANS-C01 questions linked to ANS-C01 troubleshooting.
Practice this exam
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
FAQ
Network Design — This question tests Network Design — CIDR notation defines the prefix length..
The correct answer is: /20 — Option B is correct. The total /16 has 65536 addresses. Reserving 25% leaves 49152 addresses for use. With 3 environments × 3 AZs = 9 subnets, each subnet gets 5461 addresses on average. A /19 subnet provides 8191 addresses per subnet, totaling 9×8191=73719, which exceeds the available 49152. A /20 provides 4095 addresses per subnet (9×4095=36855), which is within the available 49152 and allows efficient use. The smallest subnet that fits the requirement is /20. Option A (/18) is too large, C (/21) provides too few addresses per subnet, D (/22) also insufficient.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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.
CIDR notation defines the prefix length.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Keep practising
Last reviewed: Jun 20, 2026
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.