The answer is that the new EC2 instance was not launched with the correct security group. This is the most likely cause because security groups act as a virtual firewall at the instance level, meaning they must be explicitly assigned during instance launch or attached afterward; a security group rule cannot be applied to an instance that was never associated with that group. On the AWS Certified Security Specialty SCS-C02 exam, this question tests your understanding of the fundamental difference between security groups and network ACLs—security groups are stateful and instance-specific, while NACLs are stateless and operate at the subnet level. A common trap is confusing security group rules with default security group behavior; the default group only allows traffic from itself, not custom rules. To remember: think of security groups as a name tag you must pin on the instance at launch—no tag, no rule.
SCS-C02 Infrastructure Security Practice Question
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 has a security group that allows inbound SSH from a specific IP range. A security engineer notices that the security group rule is not being applied to a newly launched EC2 instance. What is the most likely cause?
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
✓
The new EC2 instance was not launched with the correct security group
Option B is correct because security groups are attached at the instance level; a new instance must have the security group assigned. Option A is wrong because security groups are stateful. Option C is wrong because NACLs are subnet-level, not instance-level. Option D is wrong because the default security group does not automatically include custom rules.
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 new EC2 instance was not launched with the correct security group
Why this is correct
Each instance must have the desired security group assigned.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The security group is using the default VPC security group
Why it's wrong here
The default group has different rules.
✗
The security group is configured as stateless
Why it's wrong here
Security groups are stateful by default.
✗
The network ACL is blocking SSH traffic to the subnet
Why it's wrong here
NACLs affect the subnet, not the instance's security group.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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 SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Infrastructure Security — This question tests Infrastructure Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The new EC2 instance was not launched with the correct security group — Option B is correct because security groups are attached at the instance level; a new instance must have the security group assigned. Option A is wrong because security groups are stateful. Option C is wrong because NACLs are subnet-level, not instance-level. Option D is wrong because the default security group does not automatically include custom rules.
What should I do if I get this SCS-C02 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 SCS-C02 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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Question Discussion
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