Question 255 of 519

Read, generate and modify configuration →hardMultiple ChoiceObjective-mapped

Quick Answer

The correct expression is vpc_security_group_ids = [local.security_group_map[var.subnet_ids[count.index]]]. This works because local values in Terraform are always accessed with the local. prefix, and the expression performs a map lookup using the subnet ID derived from the var.subnet_ids list at the current count.index position. On the HashiCorp Terraform Associate TF-003 exam, this question tests your understanding of local value syntax combined with count-based resource iteration and map lookups—a common scenario when mapping subnet-specific security groups. The most frequent trap is forgetting the local. prefix, as options omitting it are syntactically invalid, while others use incorrect bracket placement. Remember the memory tip: "locals always need the dot—local.name, not just name."

TF-003 Read, generate and modify configuration Practice Question

This TF-003 practice question tests your understanding of read, generate and modify configuration. 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.

Exhibit

variable "subnet_ids" {
  type = list(string)
  default = ["subnet-1", "subnet-2"]
}

resource "aws_instance" "app" {
  count         = length(var.subnet_ids)
  subnet_id     = var.subnet_ids[count.index]
  ami           = "ami-xyz"
  instance_type = "t2.micro"
}

Refer to the exhibit. You need to add a security group to each instance. You have a local value defined as 'security_group_map = { "subnet-1" = "sg-1", "subnet-2" = "sg-2" }'. Which expression should be used to reference the security group ID in the resource block?

Question 1hardmultiple choice

Review the full subnetting walkthrough →

Exhibit

variable "subnet_ids" {
  type = list(string)
  default = ["subnet-1", "subnet-2"]
}

resource "aws_instance" "app" {
  count         = length(var.subnet_ids)
  subnet_id     = var.subnet_ids[count.index]
  ami           = "ami-xyz"
  instance_type = "t2.micro"
}

A
vpc_security_group_ids = [local.security_group_map[var.subnet_ids[count.index]]]
Correctly uses local value with indexing.
B
vpc_security_group_ids = [lookup(security_group_map, var.subnet_ids[count.index])]
Why wrong: Missing 'local.' prefix; should be local.security_group_map.
C
vpc_security_group_ids = [security_group_map[var.subnet_ids[count.index]]]
Why wrong: Missing 'local.' prefix.
D
vpc_security_group_ids = [lookup(local.security_group_map, var.subnet_ids[count.index])]
Why wrong: Syntax incorrect: lookup with map and key is fine but not the intended correct answer.

Full breakdown with real-world context →

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

✓

vpc_security_group_ids = [local.security_group_map[var.subnet_ids[count.index]]]

Correct C: local values are accessed with local.name. Option A and D miss the local prefix, B is syntactically incorrect.

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.

✓
vpc_security_group_ids = [local.security_group_map[var.subnet_ids[count.index]]]
Why this is correct
Correctly uses local value with indexing.
Related concept
CIDR notation defines the prefix length.
✗
vpc_security_group_ids = [lookup(security_group_map, var.subnet_ids[count.index])]
Why it's wrong here
Missing 'local.' prefix; should be local.security_group_map.
✗
vpc_security_group_ids = [security_group_map[var.subnet_ids[count.index]]]
Why it's wrong here
Missing 'local.' prefix.
✗
vpc_security_group_ids = [lookup(local.security_group_map, var.subnet_ids[count.index])]
Why it's wrong here
Syntax incorrect: lookup with map and key is fine but not the intended correct answer.

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.

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FAQ

Questions learners often ask

What does this TF-003 question test?

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: vpc_security_group_ids = [local.security_group_map[var.subnet_ids[count.index]]] — Correct C: local values are accessed with local.name. Option A and D miss the local prefix, B is syntactically incorrect.

What should I do if I get this TF-003 question wrong?

What is the key concept behind this question?

CIDR notation defines the prefix length.

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Last reviewed: Jun 24, 2026

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