Quick Answer
The correct subnet size is /21. This is because a /21 subnet provides 2,048 total IP addresses, and after AWS reserves five addresses per subnet (for the network, VPC router, DNS, future use, and broadcast), 2,043 usable IPs remain—more than enough to host 2,000 EC2 instances. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your ability to apply subnet sizing to real-world capacity planning, often disguised as a scenario where you must choose the smallest subnet that meets a given instance count. A common trap is forgetting that AWS reserves five IPs, not just the traditional network and broadcast addresses, which can lead you to incorrectly select a /22 (which offers only 1,019 usable IPs). To remember the math, think: every subnet loses five addresses, so always subtract five from the total 2^(32-subnet bits) before comparing to your requirement.
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 want to create a subnet that can host at least 2000 EC2 instances. 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.
/21
Provides 2048 IPs, enough for 2000 instances.
/22
Why wrong: Provides 1024 IPs, insufficient.
/24
Why wrong: Only 256 IPs, insufficient.
/20
Why wrong: Provides 4096 IPs, but larger than needed.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
/21
A /21 subnet provides 2,048 usable IP addresses (2^(32-21) - 5 = 2,048 - 5 = 2,043 for AWS, but the key is that 2,048 total addresses minus the 5 reserved by AWS yields 2,043, which exceeds the requirement of 2,000 EC2 instances). AWS reserves 5 IP addresses per subnet (network, VPC router, DNS, future use, broadcast), so the usable count is total addresses minus 5. A /21 is the smallest subnet that meets the 2,000-instance requirement.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
/21
Why this is correct
Provides 2048 IPs, enough for 2000 instances.
Clue confirmation
The clue word "least" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
/22
Why it's wrong here
Provides 1024 IPs, insufficient.
/24
Why it's wrong here
Only 256 IPs, insufficient.
/20
Why it's wrong here
Provides 4096 IPs, but larger than needed.
Common exam traps
AWS often tests the misconception that you only need to consider total IP addresses (e.g., /22 gives 1,024, which seems close to 2,000) without accounting for AWS's 5 reserved addresses, or that a larger subnet like /20 is automatically better, ignoring the principle of choosing the smallest subnet that meets the requirement.
Detailed technical explanation
AWS reserves the first 4 and the last IP address in each subnet (network address, VPC router, DNS server, future use, and broadcast). This is defined in the AWS VPC documentation and is not configurable. When calculating subnet sizes for EC2 deployments, always subtract 5 from the total address count to get usable IPs. The /21 subnet (2,048 total - 5 = 2,043 usable) is the optimal choice because it provides headroom for growth without wasting address space, which is critical in large-scale VPC designs where IP exhaustion can force re-architecture.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
What to study next
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FAQ
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
The correct answer is: /21 — A /21 subnet provides 2,048 usable IP addresses (2^(32-21) - 5 = 2,048 - 5 = 2,043 for AWS, but the key is that 2,048 total addresses minus the 5 reserved by AWS yields 2,043, which exceeds the requirement of 2,000 EC2 instances). AWS reserves 5 IP addresses per subnet (network, VPC router, DNS, future use, broadcast), so the usable count is total addresses minus 5. A /21 is the smallest subnet that meets the 2,000-instance requirement.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "least". You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 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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