- A
172.16.0.0/16
Correct. 172.16.0.0/16 is in the 172.16.0.0/12 RFC 1918 range, does not overlap with 10.0.0.0/16, and is allowed as a secondary CIDR.
- B
10.0.0.0/16
Why wrong: Incorrect. 10.0.0.0/16 overlaps with the existing primary CIDR (10.0.0.0/16) and cannot be used as a secondary.
- C
10.0.0.0/8
Why wrong: Incorrect. 10.0.0.0/8 overlaps with the primary CIDR and is a larger range; it would conflict.
- D
10.1.0.0/16
Why wrong: Incorrect. Although 10.1.0.0/16 does not overlap with the primary, it is within the same 10.0.0.0/8 major network. Using a different RFC 1918 class (like 172.16.0.0/16) is recommended to simplify routing and avoid potential issues, making A the best answer.
Question 733 of 1,705
Selecting a Non-Overlapping Secondary CIDR for Your VPC
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. A key principle to apply: vPC Secondary CIDR. 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. It needs to create a secondary CIDR block for additional subnets that must not overlap with the existing CIDR. Which CIDR block should be used?
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
✓
172.16.0.0/16
Option A (172.16.0.0/16) is correct because it provides a private, non-overlapping secondary CIDR from a different RFC 1918 address class than the primary (10.0.0.0/16). AWS VPCs allow secondary CIDRs from any RFC 1918 range, regardless of the primary's class. The other options are invalid: B overlaps the primary, C overlaps and is larger, and D, while non-overlapping, is within the same 10.0.0.0/8 class as the primary and is not recommended for secondary CIDRs to avoid routing confusion.
Key principle: VPC Secondary CIDR
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
172.16.0.0/16
- ✗
10.0.0.0/16
Why it's wrong here
Incorrect. 10.0.0.0/16 overlaps with the existing primary CIDR (10.0.0.0/16) and cannot be used as a secondary.
- ✗
10.0.0.0/8
Why it's wrong here
Incorrect. 10.0.0.0/8 overlaps with the primary CIDR and is a larger range; it would conflict.
- ✗
10.1.0.0/16
Why it's wrong here
Incorrect. Although 10.1.0.0/16 does not overlap with the primary, it is within the same 10.0.0.0/8 major network. Using a different RFC 1918 class (like 172.16.0.0/16) is recommended to simplify routing and avoid potential issues, making A the best answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The ANS-C01 exam often tests the misconception that secondary CIDRs must belong to the same RFC 1918 class as the primary. In reality, AWS permits any private RFC 1918 range. The trap here is that candidates may choose D (10.1.0.0/16) thinking it matches the class, but the correct choice is A (172.16.0.0/16) which is a different class and non-overlapping.
Detailed technical explanation
How to think about this question
When adding a secondary CIDR to an existing VPC, AWS validates that the new block does not overlap with any existing CIDR and that it belongs to the same RFC 1918 address family (all 10.x.x.x, all 172.16.x.x, or all 192.168.x.x). The VPC's primary CIDR and secondary CIDRs are combined into a single routing table, and overlapping ranges would cause ambiguous routing. In practice, this allows you to expand a VPC from a /16 to multiple /16s within the 10.0.0.0/8 space, as long as they are contiguous or non-overlapping.
KKey Concepts to Remember
- VPC Secondary CIDR
- RFC 1918 Address Ranges
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
VPC Secondary CIDR
Real-world example
How this comes up in practice
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.
Visual reference
Quick reference
IPv4 Address Class Summary
| Class | First Octet Range | Default Mask | Networks | Hosts per Network |
|---|---|---|---|---|
| A | 1–126 | /8 (255.0.0.0) | 126 | 16,777,214 |
| B | 128–191 | /16 (255.255.0.0) | 16,384 | 65,534 |
| C | 192–223 | /24 (255.255.255.0) | 2,097,152 | 254 |
| D | 224–239 | N/A | Multicast groups | — |
| E | 240–255 | N/A | Reserved / experimental | — |
127.x.x.x is reserved for loopback. Modern networks use CIDR (classless) rather than classful addressing.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Design — This question tests Network Design — VPC Secondary CIDR.
What is the correct answer to this question?
The correct answer is: 172.16.0.0/16 — Option A (172.16.0.0/16) is correct because it provides a private, non-overlapping secondary CIDR from a different RFC 1918 address class than the primary (10.0.0.0/16). AWS VPCs allow secondary CIDRs from any RFC 1918 range, regardless of the primary's class. The other options are invalid: B overlaps the primary, C overlaps and is larger, and D, while non-overlapping, is within the same 10.0.0.0/8 class as the primary and is not recommended for secondary CIDRs to avoid routing confusion.
What should I do if I get this ANS-C01 question wrong?
Review vPC Secondary CIDR, then practise related ANS-C01 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
VPC Secondary CIDR
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Last reviewed: Jul 4, 2026
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