- A
10.0.0.0/20
Valid /20 subnet within the VPC.
- B
10.0.0.0/18
Why wrong: Overlaps with other subnets if used with A or D.
- C
10.0.0.0/16
Why wrong: This is the entire VPC CIDR, not a subnet.
- D
10.0.16.0/20
Valid /20 subnet adjacent to A.
- E
10.0.0.0/28
Why wrong: Too small and wasteful for expansion.
Question 94 of 1,705
ANS-C01 CIDR Allocation Practice Question
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. A key principle to apply: cIDR Allocation. 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?
Answer choices
Why each option matters
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 (10.0.0.0/20) is correct because it provides 4096 IP addresses and leaves contiguous space for other subnets, such as the /20 future expansion. Option D (10.0.16.0/20) is also correct, as it is a valid /20 subnet within the 10.0.0.0/16 VPC that can be used for future expansion without overlapping with the subnets for the three-tier application. Together, these two /20 subnets cover 10.0.0.0/19, which can be further subdivided for the application tiers across two Availability Zones.
Key principle: CIDR Allocation
Answer analysis
Option-by-option breakdown
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
CIDR Allocation
- ✗
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
CIDR Allocation
- ✗
10.0.0.0/28
Why it's wrong here
Too small and wasteful for expansion.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Candidates often pick only one valid /20 subnet (e.g., 10.0.0.0/20) but forget that 10.0.16.0/20 is also valid and efficient for future expansion. They may also incorrectly select the entire VPC CIDR (10.0.0.0/16) as a subnet or choose a /28, which is too small for production workloads.
Detailed technical explanation
How to think about this question
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.
KKey Concepts to Remember
- CIDR Allocation
- Subnet Sizing
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
CIDR Allocation
Real-world example
How this comes up in practice
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.
Visual reference
What to study next
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Design — This question tests Network Design — CIDR Allocation.
What is the correct answer to this question?
The correct answer is: 10.0.0.0/20 — Option A (10.0.0.0/20) is correct because it provides 4096 IP addresses and leaves contiguous space for other subnets, such as the /20 future expansion. Option D (10.0.16.0/20) is also correct, as it is a valid /20 subnet within the 10.0.0.0/16 VPC that can be used for future expansion without overlapping with the subnets for the three-tier application. Together, these two /20 subnets cover 10.0.0.0/19, which can be further subdivided for the application tiers across two Availability Zones.
What should I do if I get this ANS-C01 question wrong?
Review cIDR Allocation, then practise related ANS-C01 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
CIDR Allocation
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Last reviewed: Jun 24, 2026
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