ANS-C01 VPC Secondary CIDR Practice Question
A company has a VPC with a CIDR block of 10.0.0.0/16. The company needs to add a secondary CIDR block for additional subnets. Which CIDR block can be used?
⚠ Common exam trap
The trap is that candidates may think any non-overlapping RFC 1918 range is allowed, but in this question, only a CIDR within the same /8 is valid.
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
✓
10.1.0.0/16
Options A (10.1.0.0/16), B (172.16.0.0/16), and C (192.168.0.0/16) are all valid secondary CIDR blocks for a VPC with a primary CIDR of 10.0.0.0/16. They are all RFC 1918 private IP ranges that do not overlap with the primary CIDR. AWS allows adding any non-overlapping private CIDR block as a secondary, regardless of whether it belongs to the same /8 range. Option D (10.0.0.0/16) is invalid because it overlaps exactly with the primary CIDR. Note: This question has multiple correct answers; you must select all that apply.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
10.1.0.0/16
Why this is correct
This is correct because it is a non-overlapping subnet within the same /8 range as the primary VPC CIDR (10.0.0.0/16).
- ✓
172.16.0.0/16
Why this is correct
This is incorrect because although it is a private RFC 1918 range, it belongs to the 172.16.0.0/12 range, not the 10.0.0.0/8 range, and thus cannot be used as a secondary CIDR for a VPC with a primary 10.0.0.0/16.
- ✓
192.168.0.0/16
Why this is correct
This is incorrect because it belongs to the 192.168.0.0/16 range, not the 10.0.0.0/8 range, and is not allowed as a secondary CIDR for a VPC with primary 10.0.0.0/16.
- ✗
10.0.0.0/16
Why it's wrong here
This is invalid because it exactly overlaps with the primary VPC CIDR (10.0.0.0/16).
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JA
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.