ANS-C01 Network Design Practice Question
A company has a VPC with a CIDR block of 10.0.0.0/16. They need to add a new CIDR block 10.1.0.0/16 to accommodate more subnets. The VPC already has multiple subnets and route tables. What is the impact of adding this secondary CIDR?
⚠ Common exam trap
AWS often tests the misconception that adding a secondary CIDR automatically propagates to route tables or extends existing subnets, when in fact it requires manual route table updates and new subnet creation.
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
✓
Existing subnets and route tables remain unchanged; new subnets can be created in the new CIDR.
Adding a secondary CIDR block to an existing VPC is a non-disruptive operation. The existing subnets and route tables remain completely unchanged because the secondary CIDR is an additional IP address space, not a modification of the existing one. New subnets can be created within the new CIDR block, and you must explicitly associate them with route tables as needed.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The secondary CIDR will be automatically advertised to all route tables.
Why it's wrong here
You must explicitly add routes for the secondary CIDR in route tables.
- ✓
Existing subnets and route tables remain unchanged; new subnets can be created in the new CIDR.
Why this is correct
Secondary CIDRs do not affect existing resources.
- ✗
The VPC will experience a brief outage while the new CIDR is being associated.
Why it's wrong here
Adding a secondary CIDR is a non-disruptive operation.
- ✗
Existing subnets will automatically be extended to include IPs from the new CIDR.
Why it's wrong here
Existing subnets remain in their original CIDR; new subnets must be created in the new CIDR.
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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 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?
medium- ✓ A.10.1.0.0/16
- ✓ B.172.16.0.0/16
- ✓ C.192.168.0.0/16
- D.10.0.0.0/16
Why A: 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.
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.