ANS-C01 Network Design Practice Question
A company has a VPC with an IPv4 CIDR block of 10.0.0.0/16. They need to add additional IP address space for new subnets. The existing subnets use 10.0.0.0/17 and 10.0.128.0/17. Which CIDR block should be added as a secondary CIDR to the VPC to provide the most additional contiguous address space?
⚠ Common exam trap
The trap here is that candidates mistakenly think they can reuse part of the same /16 block (like 10.0.0.0/16 again) or use a larger block like /8, but AWS requires secondary CIDRs to be non-overlapping and within the /16 maximum for a single VPC.
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
(10.1.0.0/16) is correct because the VPC already uses the entire 10.0.0.0/16 range (split into 10.0.0.0/17 and 10.0.128.0/17). To add contiguous address space, you must use a different /16 block that is adjacent in the private IPv4 address space, such as 10.1.0.0/16. AWS VPCs support secondary CIDRs that must not overlap with existing CIDRs, and 10.1.0.0/16 provides a non-overlapping, contiguous block.
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 a non-overlapping /16 block contiguous to the existing /16, providing additional address space.
- ✗
10.0.0.0/0
Why it's wrong here
This is an invalid CIDR block for a VPC.
- ✗
10.0.0.0/8
Why it's wrong here
This overlaps with the existing 10.0.0.0/16 and is not allowed as a secondary CIDR because it is larger than /16.
- ✗
10.0.0.0/16
Why it's wrong here
This is the same as the primary CIDR and cannot be added.
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Same concept, more angles
1 more way this is tested on ANS-C01
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company has a VPC with an IPv4 CIDR of 10.0.0.0/16. It needs to add an additional non-overlapping CIDR for new workloads. Which CIDR should be used?
easy- A.10.0.0.0/24
- ✓ B.10.1.0.0/16
- C.10.0.1.0/24
- D.10.0.0.0/8
Why B: (10.1.0.0/16) is correct because it provides a non-overlapping CIDR block that does not conflict with the existing VPC CIDR of 10.0.0.0/16. In AWS, when adding a secondary CIDR to a VPC, the new block must not overlap with the existing VPC CIDR or any of its subnets. The 10.1.0.0/16 range is entirely separate from 10.0.0.0/16, satisfying this requirement.
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.