Quick Answer
The correct subnet CIDR blocks are /20 and /21, as both provide at least 2000 IP addresses for the container cluster. A /21 subnet offers 2^(32-21) = 2048 total IP addresses, and a /20 offers 4096, each comfortably exceeding the 2000-IP requirement even after AWS reserves five addresses per subnet. This question tests your ability to perform subnet CIDR sizing for container clusters, a common scenario on the AWS Certified Advanced Networking Specialty ANS-C01 exam where you must match IP capacity to workload demands. The trap is confusing total IPs with usable IPs—AWS reserves the first four and last one, so a /21 yields 2043 usable, still well above 2000. For a memory tip, remember that each increment in CIDR prefix (e.g., /21 to /20) doubles the IP count, so when you need at least 2000, target /21 or larger (smaller prefix number).
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. The company needs to add a new subnet for a container cluster that requires at least 2000 IP addresses. Which TWO subnet CIDR blocks meet this requirement? (Choose two.)
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.
10.0.0.0/24
Why wrong: Only 256 addresses.
10.0.0.0/25
Why wrong: 128 addresses.
10.0.0.0/20
4096 addresses, more than 2000.
10.0.0.0/21
2048 addresses, meets requirement.
10.0.0.0/26
Why wrong: 64 addresses.
Answer choices
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
Option C (10.0.0.0/20) provides 2^(32-20) = 4096 IP addresses, and option D (10.0.0.0/21) provides 2^(32-21) = 2048 IP addresses. Both meet the requirement of at least 2000 usable IP addresses (after subtracting AWS reserved addresses, each still offers well over 2000).
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.
10.0.0.0/24
Why it's wrong here
Only 256 addresses.
10.0.0.0/25
Why it's wrong here
128 addresses.
10.0.0.0/20
Why this is correct
4096 addresses, more than 2000.
Clue confirmation
The clue word "least" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
10.0.0.0/21
Why this is correct
2048 addresses, meets requirement.
Clue confirmation
The clue word "least" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
10.0.0.0/26
Why it's wrong here
64 addresses.
Common exam traps
AWS often tests the misconception that a /24 subnet (256 addresses) is sufficient for 2000 IPs, or that candidates forget to calculate total addresses as 2^(32-prefix) and instead guess based on the prefix number alone.
Detailed technical explanation
In AWS, each subnet reserves 5 IP addresses for internal use (network, VPC router, DNS, future use, broadcast), so the usable count is total minus 5. For a /21 subnet, 2048 - 5 = 2043 usable addresses, which still exceeds 2000. The CIDR block must be a subset of the VPC's 10.0.0.0/16 range, and all options here are valid subnets of that range, but only /20 and /21 provide enough capacity.
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: 10.0.0.0/20 — Option C (10.0.0.0/20) provides 2^(32-20) = 4096 IP addresses, and option D (10.0.0.0/21) provides 2^(32-21) = 2048 IP addresses. Both meet the requirement of at least 2000 usable IP addresses (after subtracting AWS reserved addresses, each still offers well over 2000).
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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