ANS-C01 Network Design Practice Question
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?
⚠ Common exam trap
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.
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
✓
/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.
Answer analysis
Option-by-option breakdown
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.
- ✗
/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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.