Quick Answer
The correct subnet CIDR allocation is 10.0.16.0/20, as it provides a contiguous block of 4096 IP addresses within the 10.0.0.0/16 VPC while leaving room for the required three-tier subnets across two Availability Zones and a separate /20 for future expansion. This is valid because VPC subnet CIDR allocation must not overlap, and starting the future /20 at 10.0.16.0 preserves the 10.0.0.0/20 range for the application tier, allowing efficient use of the address space without fragmentation. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this tests your ability to plan hierarchical IP addressing under constraints like multi-AZ distribution and reserved capacity, often appearing in scenario-based questions where overlapping or non-contiguous allocations are common traps. A key memory tip is to think of the VPC as a pizza: slice the largest reserved piece first (the /20 expansion), then carve the remaining contiguous slices for each tier, ensuring no slice overlaps another.
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 of 10.0.0.0/16. They need to create subnets for a three-tier application (web, application, database) across two Availability Zones. They also need a /20 subnet for a future expansion. Which TWO subnet CIDR allocations are valid and efficient?
10.0.0.0/20
Valid /20 subnet within the VPC.
10.0.0.0/18
Why wrong: Overlaps with other subnets if used with A or D.
10.0.0.0/16
Why wrong: This is the entire VPC CIDR, not a subnet.
10.0.16.0/20
Valid /20 subnet adjacent to A.
10.0.0.0/28
Why wrong: Too small and wasteful for expansion.
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 A is correct because 10.0.0.0/20 is a valid subnet within the 10.0.0.0/16 VPC, providing 4096 IP addresses. It is an efficient allocation as it leaves contiguous space for the other required subnets (e.g., /20 for future expansion) without overlapping, and it aligns with the need for subnets across two Availability Zones.
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.
10.0.0.0/20
Why this is correct
Valid /20 subnet within the VPC.
Related concept
Read the scenario before looking for a memorised answer.
10.0.0.0/18
Why it's wrong here
Overlaps with other subnets if used with A or D.
10.0.0.0/16
Why it's wrong here
This is the entire VPC CIDR, not a subnet.
10.0.16.0/20
Why this is correct
Valid /20 subnet adjacent to A.
Related concept
Read the scenario before looking for a memorised answer.
10.0.0.0/28
Why it's wrong here
Too small and wasteful for expansion.
Common exam traps
The trap here is that candidates often confuse the VPC CIDR with a subnet CIDR, selecting the entire /16 as a subnet, or they fail to recognize that a /20 subnet like 10.0.16.0/20 is valid and efficient for future expansion, while smaller subnets like /28 are impractical for production workloads.
Detailed technical explanation
In AWS VPC design, subnet CIDRs must be non-overlapping and fully contained within the VPC CIDR. The /20 subnet (10.0.16.0/20) is a valid choice for the future expansion because it falls within the 10.0.0.0/16 range and does not conflict with other subnets when planned correctly. Under the hood, AWS uses a hierarchical IP addressing scheme where contiguous CIDR blocks simplify routing and avoid fragmentation, which is critical for scalable multi-tier architectures across Availability Zones.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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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 A is correct because 10.0.0.0/20 is a valid subnet within the 10.0.0.0/16 VPC, providing 4096 IP addresses. It is an efficient allocation as it leaves contiguous space for the other required subnets (e.g., /20 for future expansion) without overlapping, and it aligns with the need for subnets across two Availability Zones.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 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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