Quick Answer
The answer is a /20 subnet. This is correct because a /20 provides 4,096 total IP addresses, and after AWS reserves five for internal use, you have 4,091 usable IPs—well above the required 2,000. The key concept here is that subnet size calculation for usable IPs must account for the AWS-specific reservation of five IPs per subnet (network, gateway, two for DNS, and one reserved), not the standard two reserved in traditional networking. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your ability to quickly match business requirements to the smallest sufficient CIDR block, avoiding the common trap of choosing a /22 (which yields only 1,023 usable IPs) or oversizing with a /19. A useful memory tip: for every 10 bits in the host portion, you get roughly 1,000 usable IPs—so a /20 (12 host bits) gives about 4,000, easily covering 2,000.
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 has a VPC with a CIDR of 10.0.0.0/16. They need to create a subnet for a new application that requires at least 2000 usable IP addresses. Which subnet size should they choose?
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.
/24 (256 IPs)
Why wrong: Only 251 usable, insufficient.
/22 (1024 IPs)
Why wrong: 1023 usable, insufficient.
/19 (8192 IPs)
Why wrong: Larger than necessary but could work; however, /20 is the most efficient.
/20 (4096 IPs)
4091 usable, sufficient.
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 (4096 IPs)
Option C is correct. A /20 subnet provides 4096 total IPs, with 4091 usable (AWS reserves 5). For 2000 usable, /20 is sufficient. Option A (/24) provides 251 usable. Option B (/22) provides 1023 usable. Option D (/19) provides 8191 usable, which is more than needed but acceptable, though /20 is the smallest sufficient.
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.
/24 (256 IPs)
Why it's wrong here
Only 251 usable, insufficient.
/22 (1024 IPs)
Why it's wrong here
1023 usable, insufficient.
/19 (8192 IPs)
Why it's wrong here
Larger than necessary but could work; however, /20 is the most efficient.
/20 (4096 IPs)
Why this is correct
4091 usable, sufficient.
Clue confirmation
The clue word "least" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
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
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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.
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FAQ
Network Design — This question tests Network Design — CIDR notation defines the prefix length..
The correct answer is: /20 (4096 IPs) — Option C is correct. A /20 subnet provides 4096 total IPs, with 4091 usable (AWS reserves 5). For 2000 usable, /20 is sufficient. Option A (/24) provides 251 usable. Option B (/22) provides 1023 usable. Option D (/19) provides 8191 usable, which is more than needed but acceptable, though /20 is the smallest sufficient.
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". 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.
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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.
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