Quick Answer
The answer is ec2:RunInstances, because this is the specific IAM action that controls the ability to launch new EC2 instances, and it supports the ec2:Subnet condition key to restrict which subnet can be used. When you attach a policy with a condition like "ec2:Subnet" set to a specific subnet ARN, the developer can only launch instances into that subnet, and any attempt to use another subnet will be denied. On the AWS Certified Security Specialty SCS-C02 exam, this tests your understanding of service-specific condition keys and how they enforce granular resource-level permissions—a common trap is confusing the launch action with read-only actions like ec2:DescribeSubnets or tagging actions like ec2:CreateTags. Remember, to control where an instance is placed, you must condition the action that creates it, not the actions that describe or modify subnets. A useful memory tip: think "RunInstances runs the launch, so condition the subnet on the run."
This SCS-C02 practice question tests your understanding of infrastructure security. 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 wants to allow a developer to launch EC2 instances only in a specific subnet. The developer should not be able to use any other subnet. Which IAM policy action should be used to enforce this?
ec2:ModifySubnetAttribute
Why wrong: This modifies subnet attributes, not launching instances.
ec2:CreateTags
Why wrong: This is used for tagging resources.
ec2:RunInstances
This action launches instances and can be restricted by subnet.
ec2:DescribeSubnets
Why wrong: This is a read-only action for listing subnets.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
ec2:RunInstances
Option A is correct because 'ec2:RunInstances' is the action for launching instances, and a condition on 'ec2:Subnet' can restrict the subnet. Option B is wrong because 'ec2:CreateTags' is for tagging. Option C is wrong because 'ec2:DescribeSubnets' is read-only. Option D is wrong because 'ec2:ModifySubnetAttribute' is for modifying subnets.
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
For each option: why learners choose it and why it is or isn't the right answer here.
ec2:ModifySubnetAttribute
Why it's wrong here
This modifies subnet attributes, not launching instances.
ec2:CreateTags
Why it's wrong here
This is used for tagging resources.
ec2:RunInstances
Why this is correct
This action launches instances and can be restricted by subnet.
Related concept
CIDR notation defines the prefix length.
ec2:DescribeSubnets
Why it's wrong here
This is a read-only action for listing subnets.
Common exam traps
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
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.
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
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Infrastructure Security — This question tests Infrastructure Security — CIDR notation defines the prefix length..
The correct answer is: ec2:RunInstances — Option A is correct because 'ec2:RunInstances' is the action for launching instances, and a condition on 'ec2:Subnet' can restrict the subnet. Option B is wrong because 'ec2:CreateTags' is for tagging. Option C is wrong because 'ec2:DescribeSubnets' is read-only. Option D is wrong because 'ec2:ModifySubnetAttribute' is for modifying subnets.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
CIDR notation defines the prefix length.
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Last reviewed: Jun 20, 2026
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