TF-003 Read, generate and modify configuration Practice Question
This TF-003 practice question tests your understanding of read, generate and modify configuration. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 team uses Terraform to manage multiple environments (dev, staging, prod) with a shared networking module. The module defines a variable 'cidr_block' with no default. In the root module, they have a file dev.tfvars containing 'cidr_block = "10.0.0.0/16"'. When running 'terraform plan' while in the dev workspace, they receive: 'Error: No value for required variable cidr_block'. They have already run 'terraform init' and confirmed the workspace is 'dev'. What is the most likely cause and correct action?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
They forgot to include the -var-file flag; add -var-file='dev.tfvars' to the plan command.
The error indicates the variable file is not being loaded. The most common reason is that the -var-file flag was omitted. Option B fixes this by explicitly specifying the variable file. Option A is incorrect because the variable is in the module, and root module can pass it via module block, but the variable is already defined in the module. Option C is incorrect because passing via module block is the correct way, but the error suggests the variable file is not being read at all. Option D is incorrect because the workspace is already set correctly.
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.
✓
They forgot to include the -var-file flag; add -var-file='dev.tfvars' to the plan command.
Why this is correct
Terraform does not automatically load arbitrary .tfvars files; using -var-file explicitly loads it.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The variable is defined in the child module; they need to reference it with module.cidr_block in the root module.
Why it's wrong here
Variables are accessed by name; no need to prefix with module.
✗
The workspace is not selected; run 'terraform workspace select dev' again.
Why it's wrong here
The user already confirmed the workspace is dev.
✗
The variable must be passed through the module block; they should add a module input assignment.
Why it's wrong here
The module already expects the variable; passing via -var-file should work if the file is loaded.
Common exam traps
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: They forgot to include the -var-file flag; add -var-file='dev.tfvars' to the plan command. — The error indicates the variable file is not being loaded. The most common reason is that the -var-file flag was omitted. Option B fixes this by explicitly specifying the variable file. Option A is incorrect because the variable is in the module, and root module can pass it via module block, but the variable is already defined in the module. Option C is incorrect because passing via module block is the correct way, but the error suggests the variable file is not being read at all. Option D is incorrect because the workspace is already set correctly.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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