Quick Answer
The correct choice is 10.1.0.0/16 because it is a non-overlapping RFC 1918 private IP range that sits entirely outside the existing VPC CIDR of 10.0.0.0/16. When selecting a valid secondary CIDR for a VPC without overlapping, the key requirement is that the new block must not share any IP addresses with the primary or any existing secondary blocks—AWS will reject any overlap, even if the ranges are within the same 10.0.0.0/8 supernet. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this concept tests your understanding of VPC CIDR design and the constraints of secondary IP allocation; a common trap is assuming any /16 within 10.0.0.0/8 works, but overlapping ranges like 10.0.0.0/16 or 10.0.128.0/17 would be invalid. A useful memory tip: think of the secondary CIDR as needing a completely different second octet—if your primary is 10.0.x.x, choose 10.1.x.x or higher to guarantee no overlap.
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. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. 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 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?
10.1.0.0/16
Non-overlapping private IP range.
172.16.0.0/16
Why wrong: 172.16.0.0/12 is private, but this specific range may be acceptable; however, 10.1.0.0/16 is simpler.
192.168.0.0/16
Why wrong: 192.168.0.0/16 is private, but 10.1.0.0/16 is also private and more straightforward.
10.0.0.0/16
Why wrong: Overlaps with the existing CIDR.
Answer choices
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
Option A (10.1.0.0/16) is correct because it is a valid RFC 1918 private IP address range that does not overlap with the existing VPC CIDR block (10.0.0.0/16). AWS allows adding a secondary CIDR block to a VPC as long as it does not conflict with the primary CIDR or any existing secondary CIDR blocks, and 10.1.0.0/16 is a non-overlapping /16 within the 10.0.0.0/8 private range.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
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
Non-overlapping private IP range.
Related concept
Read the scenario before looking for a memorised answer.
172.16.0.0/16
Why it's wrong here
172.16.0.0/12 is private, but this specific range may be acceptable; however, 10.1.0.0/16 is simpler.
192.168.0.0/16
Why it's wrong here
192.168.0.0/16 is private, but 10.1.0.0/16 is also private and more straightforward.
10.0.0.0/16
Why it's wrong here
Overlaps with the existing CIDR.
Common exam traps
AWS often tests the misconception that any RFC 1918 private IP range is acceptable as a secondary CIDR, but the trap here is that the question implicitly expects the secondary CIDR to be within the same /8 as the primary to avoid routing issues, and candidates may incorrectly choose 172.16.0.0/16 or 192.168.0.0/16 without considering the need for non-overlapping and contiguous addressing.
Detailed technical explanation
When adding a secondary CIDR to an Amazon VPC, the new block must not overlap with any existing CIDR blocks in the VPC or any connected networks (e.g., VPN, Direct Connect, VPC peering). AWS supports secondary CIDRs from any RFC 1918 range, but using a contiguous /16 within the same /8 (e.g., 10.1.0.0/16) is a common best practice to simplify route summarization and avoid fragmentation. The VPC's primary CIDR cannot be changed after creation, so the secondary CIDR must be a distinct, non-overlapping block.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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FAQ
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
The correct answer is: 10.1.0.0/16 — Option A (10.1.0.0/16) is correct because it is a valid RFC 1918 private IP address range that does not overlap with the existing VPC CIDR block (10.0.0.0/16). AWS allows adding a secondary CIDR block to a VPC as long as it does not conflict with the primary CIDR or any existing secondary CIDR blocks, and 10.1.0.0/16 is a non-overlapping /16 within the 10.0.0.0/8 private range.
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Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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