The answer is to use the Fn::Cidr function with the stack ID as a seed to generate a unique /16 CIDR from a larger pool. This is correct because Fn::Cidr performs dynamic CIDR allocation in CloudFormation by taking a base CIDR block, a count, and a CIDR mask, then deterministically producing non-overlapping subnets; when you pass the stack ID as the seed, each stack instance generates a unique, repeatable range without manual intervention. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this question tests your understanding of CloudFormation’s intrinsic functions for network automation, often appearing as a scenario where you must avoid overlapping VPCs across accounts and regions. A common trap is confusing Fn::Cidr with Ref or Fn::GetAtt, which return resource IDs or attributes, not new CIDR blocks. Memory tip: think “Cidr + Seed = Unique CIDR” — the seed ensures each stack gets its own slice of the pool.
DOP-C02 Configuration Management and IaC Practice Question
This DOP-C02 practice question tests your understanding of configuration management and iac. 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 uses a central CloudFormation template to create VPCs with a standard CIDR block of 10.0.0.0/16. The template is used across multiple accounts and regions. The team needs to ensure that the VPC CIDR does not overlap with other VPCs in the same account. Which approach should the engineer take to dynamically assign a unique /16 subnet from a larger pool?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use the Fn::Cidr function with the stack ID as a seed to generate a /16 CIDR from a larger pool.
Option A is correct because AWS CloudFormation's Cidr function can allocate subnets from a larger CIDR pool, and since it uses the stack ID as a seed, it produces a deterministic but unique range per stack. Option B is wrong because the Ref function returns the VPC ID, not a CIDR. Option C is wrong because Fn::GetAtt on a VPC returns attributes like VpcId, not a new CIDR. Option D is wrong because Fn::Select on a list of CIDRs requires manual maintenance and is not dynamic.
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
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Use Fn::Select on a predefined list of CIDR blocks passed as a parameter.
Why it's wrong here
D: This requires manual management of the list and may cause overlap if not tracked.
✗
Use Fn::GetAtt to retrieve the VPC's CidrBlock from another VPC resource.
Common exam trap: usable hosts are not the same as total addresses
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
How to think about this question
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.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
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
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.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DOP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Configuration Management and IaC — This question tests Configuration Management and IaC — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Use the Fn::Cidr function with the stack ID as a seed to generate a /16 CIDR from a larger pool. — Option A is correct because AWS CloudFormation's Cidr function can allocate subnets from a larger CIDR pool, and since it uses the stack ID as a seed, it produces a deterministic but unique range per stack. Option B is wrong because the Ref function returns the VPC ID, not a CIDR. Option C is wrong because Fn::GetAtt on a VPC returns attributes like VpcId, not a new CIDR. Option D is wrong because Fn::Select on a list of CIDRs requires manual maintenance and is not dynamic.
What should I do if I get this DOP-C02 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DOP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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