Quick Answer
The correct answer is 10.1.0.0/16 because it provides the most additional contiguous address space without overlapping the existing VPC CIDR of 10.0.0.0/16. When adding a secondary CIDR to a VPC, the new block must be non-overlapping with the primary CIDR and any existing subnets; here, the existing subnets already consume 10.0.0.0/17 and 10.0.128.0/17, so the next contiguous private block that does not overlap is 10.1.0.0/16, which adds a full /16 of usable IPs. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this tests your understanding of VPC CIDR design and the rule that secondary CIDRs must be from the same RFC 1918 range but cannot overlap with any existing CIDR or subnet. A common trap is choosing 10.0.0.0/8, which overlaps, or 10.0.0.0/16, which duplicates the primary. Remember the memory tip: “If the primary is a /16, the next contiguous /16 is one network over—just increment the second octet.”
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 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?
10.1.0.0/16
This is a non-overlapping /16 block contiguous to the existing /16, providing additional address space.
10.0.0.0/0
Why wrong: This is an invalid CIDR block for a VPC.
10.0.0.0/8
Why wrong: 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 wrong: This is the same as the primary CIDR and cannot be added.
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 C is correct because 10.1.0.0/16 is a contiguous block that does not overlap with the existing CIDR and adds a full /16 of address space. Option A is wrong because 10.0.0.0/8 overlaps with the existing CIDR and is too large. Option B is wrong because 10.0.0.0/16 duplicates the existing CIDR. Option D is wrong because 10.0.0.0/0 is invalid.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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
This is a non-overlapping /16 block contiguous to the existing /16, providing additional address space.
Related concept
CIDR notation defines the prefix length.
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.
Common exam traps
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
What to study next
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Network Design — This question tests Network Design — CIDR notation defines the prefix length..
The correct answer is: 10.1.0.0/16 — Option C is correct because 10.1.0.0/16 is a contiguous block that does not overlap with the existing CIDR and adds a full /16 of address space. Option A is wrong because 10.0.0.0/8 overlaps with the existing CIDR and is too large. Option B is wrong because 10.0.0.0/16 duplicates the existing CIDR. Option D is wrong because 10.0.0.0/0 is invalid.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
CIDR notation defines the prefix length.
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Same concept, more angles
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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?
easyWhy B: Option 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.
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Last reviewed: Jun 20, 2026
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