TF-003 Understand Terraform basics Practice Question
This TF-003 practice question tests your understanding of understand terraform basics. 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.
Exhibit
terraform plan
Terraform will perform the following actions:
# aws_security_group.web_sg will be updated in-place
~ resource "aws_security_group" "web_sg" {
id = "sg-12345678"
~ ingress {
- cidr_blocks = ["0.0.0.0/0"]
+ cidr_blocks = ["10.0.0.0/8"]
}
}
Plan: 0 to add, 1 to change, 0 to destroy.
Refer to the exhibit. An engineer runs terraform plan and sees this output. Which statement about the planned change is true?
Exhibit
terraform plan
Terraform will perform the following actions:
# aws_security_group.web_sg will be updated in-place
~ resource "aws_security_group" "web_sg" {
id = "sg-12345678"
~ ingress {
- cidr_blocks = ["0.0.0.0/0"]
+ cidr_blocks = ["10.0.0.0/8"]
}
}
Plan: 0 to add, 1 to change, 0 to destroy.
A
The change will replace the existing ingress rule with a new one.
The plan output shows '-/+' next to the ingress rule, meaning Terraform will destroy the existing rule and create a new one with the updated CIDR block. This is a replacement, not an in-place update.
B
The security group will be destroyed and recreated.
Why wrong: The plan shows an update in-place, not destroy and recreate.
C
Terraform will add a new ingress rule without removing the old one.
Why wrong: The plan shows a modification, not an addition; the minus and plus signs indicate replace.
D
The security group will be updated but the ingress rule will remain unchanged.
Why wrong: The plan clearly shows changes to the ingress rule.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The change will replace the existing ingress rule with a new one.
The plan output shows a -/+ next to the ingress rule, indicating that Terraform will destroy the existing rule and create a new one with the updated CIDR block. This replacement occurs because the `cidr_blocks` attribute change for an ingress rule forces resource recreation. Thus, the existing ingress rule is replaced with a new one, matching option A.
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.
✓
The change will replace the existing ingress rule with a new one.
Why this is correct
The plan output shows '-/+' next to the ingress rule, meaning Terraform will destroy the existing rule and create a new one with the updated CIDR block. This is a replacement, not an in-place update.
The security group will be destroyed and recreated.
Why it's wrong here
The plan shows an update in-place, not destroy and recreate.
✗
Terraform will add a new ingress rule without removing the old one.
Why it's wrong here
The plan shows a modification, not an addition; the minus and plus signs indicate replace.
✗
The security group will be updated but the ingress rule will remain unchanged.
Why it's wrong here
The plan clearly shows changes to the ingress rule.
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.
Trap categories for this question
Command / output trap
The plan shows an update in-place, not destroy and recreate.
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.
Visual reference
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.
Understand Terraform basics — This question tests Understand Terraform basics — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The change will replace the existing ingress rule with a new one. — The plan output shows a -/+ next to the ingress rule, indicating that Terraform will destroy the existing rule and create a new one with the updated CIDR block. This replacement occurs because the `cidr_blocks` attribute change for an ingress rule forces resource recreation. Thus, the existing ingress rule is replaced with a new one, matching option A.
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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