The correct answer is that changing the for_each key results in the destruction of the existing subnet and the creation of a new one. This occurs because Terraform uses the map keys from the for_each expression as unique resource addresses in the state file; when you modify a key, Terraform interprets it as the removal of the old address and the introduction of a completely new resource, rather than an in-place update. On the HashiCorp Terraform Associate TF-003 exam, this concept tests your understanding of how for_each key change behavior differs from modifying resource arguments, often appearing as a trap where candidates mistakenly expect a simple update. A common memory tip is to think of the for_each key as a resource’s identity card—changing the card number means the old resource is discarded and a new one is built from scratch.
TF-003 Read, generate and modify configuration Practice Question
This TF-003 practice question tests your understanding of read, generate and modify configuration. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 Terraform configuration uses a for_each expression to create multiple subnets. After applying, one subnet's configuration needs to be modified. The engineer updates the resource block's map key for that subnet. What will terraform plan show?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Destruction of the existing subnet and creation of a new one.
Option D is correct: changing the map key results in destruction of the old subnet and creation of a new one because for_each uses keys as resource addresses. Option A is wrong because the key change is not an in-place update. Option B is wrong because it only shows deletion, not creation. Option C is wrong because it suggests no changes.
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.
✗
No changes because the subnet configuration is the same.
Why it's wrong here
The resource address changed, so Terraform sees a new resource and an old one to destroy.
✗
Only deletion of the old subnet.
Why it's wrong here
Terraform will also create a new subnet with the new key.
✗
An in-place update to the subnet.
Why it's wrong here
Changing the for_each key always results in destroy+create.
✓
Destruction of the existing subnet and creation of a new one.
Why this is correct
The map key is part of the resource address; changing it causes destroy+create.
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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
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 TF-003 subnetting questions on CIDR, address ranges, and subnet selection.
Read, generate and modify configuration — This question tests Read, generate and modify configuration — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Destruction of the existing subnet and creation of a new one. — Option D is correct: changing the map key results in destruction of the old subnet and creation of a new one because for_each uses keys as resource addresses. Option A is wrong because the key change is not an in-place update. Option B is wrong because it only shows deletion, not creation. Option C is wrong because it suggests no changes.
What should I do if I get this TF-003 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 TF-003 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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