Quick Answer
The correct answer is 10.0.3.0/24, along with 10.0.0.0/24, as both are valid subnet CIDRs that fall within the VPC CIDR 10.0.0.0/16 and do not overlap with the existing 10.0.1.0/24 and 10.0.2.0/24 subnets. The key technical concept here is non-overlapping CIDR allocation: each subnet must have a unique IP range that is a subset of the VPC’s larger block, and the /24 prefix length ensures the new subnet’s third octet is distinct from the occupied ranges (1 and 2), so 10.0.3.0/24 and 10.0.0.0/24 both satisfy this condition. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this tests your ability to apply VPC subnetting fundamentals under the “Network Design” domain, often appearing in scenario-based questions where you must identify valid CIDRs while avoiding overlap. A common trap is assuming that only sequential octets (like .3) are valid, but 10.0.0.0/24 is also non-overlapping because it starts at the VPC’s base address and does not conflict with .1 or .2. Memory tip: think of subnet CIDRs like parking spots in a lot—each spot must be a distinct, non-overlapping space within the lot’s total area, and you can use any empty spot, not just the next one in line.
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 CIDR 10.0.0.0/16 and two subnets: 10.0.1.0/24 (public) and 10.0.2.0/24 (private). The company wants to add a new subnet for a third tier. Which of the following are valid subnet CIDRs that can be added? (Select TWO.)
10.0.1.128/25
Why wrong: This overlaps with the existing public subnet 10.0.1.0/24.
10.0.3.0/24
This is within the VPC CIDR and does not overlap with existing subnets.
10.0.0.0/24
This is within the VPC CIDR and does not overlap; it is a new block.
10.1.0.0/16
Why wrong: This is outside the VPC CIDR 10.0.0.0/16.
10.0.2.64/26
Why wrong: This overlaps with the existing private subnet 10.0.2.0/24.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
10.0.3.0/24
Option B (10.0.3.0/24) is correct because it falls within the VPC CIDR 10.0.0.0/16 and does not overlap with the existing subnets (10.0.1.0/24 and 10.0.2.0/24). The /24 prefix length matches the existing subnet structure, and the third octet (3) is outside the range of the first two subnets, ensuring no IP address conflict.
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.
10.0.1.128/25
Why it's wrong here
This overlaps with the existing public subnet 10.0.1.0/24.
10.0.3.0/24
Why this is correct
This is within the VPC CIDR and does not overlap with existing subnets.
Related concept
Read the scenario before looking for a memorised answer.
10.0.0.0/24
Why this is correct
This is within the VPC CIDR and does not overlap; it is a new block.
Related concept
Read the scenario before looking for a memorised answer.
10.1.0.0/16
Why it's wrong here
This is outside the VPC CIDR 10.0.0.0/16.
10.0.2.64/26
Why it's wrong here
This overlaps with the existing private subnet 10.0.2.0/24.
Common exam traps
AWS often tests the misconception that a subnet can be a subset of an existing subnet (e.g., 10.0.1.128/25 within 10.0.1.0/24), but AWS explicitly prohibits overlapping CIDR blocks within a VPC.
Detailed technical explanation
AWS VPC subnets must be non-overlapping and fully contained within the VPC CIDR. The subnet CIDR must not be a subset of an existing subnet; AWS uses a hierarchical IP addressing model where each subnet is a distinct block. In practice, when designing multi-tier architectures, you must allocate contiguous or non-overlapping CIDR blocks (e.g., 10.0.3.0/24) to avoid routing conflicts and ensure proper network segmentation.
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.
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: 10.0.3.0/24 — Option B (10.0.3.0/24) is correct because it falls within the VPC CIDR 10.0.0.0/16 and does not overlap with the existing subnets (10.0.1.0/24 and 10.0.2.0/24). The /24 prefix length matches the existing subnet structure, and the third octet (3) is outside the range of the first two subnets, ensuring no IP address conflict.
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 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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