The correct answer is security groups and network ACLs (NACLs). Security groups are the primary, stateful firewall that controls inbound traffic directly at the EC2 instance level, automatically allowing return traffic regardless of outbound rules, while NACLs operate as a stateless firewall at the subnet boundary, requiring explicit rules for both inbound and outbound traffic. On the AWS Certified Security Specialty SCS-C02 exam, this distinction tests your understanding of the shared responsibility model and network defense layers, often appearing in scenario-based questions where you must choose the right control for a specific traffic flow. A common trap is confusing IAM policies or CloudWatch alarms with network filtering, but remember that IAM governs identities, not packets, and CloudWatch only monitors. For the exam, keep this memory tip handy: security groups are “stateful and instance-level,” while NACLs are “stateless and subnet-level”—think of the S in security groups for “sticky” state, and the N in NACL for “network boundary.”
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.
Which TWO of the following are valid ways to control inbound traffic to an EC2 instance? (Select TWO.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Network ACLs
Options A and D are correct because security groups are stateful firewalls for instances, and NACLs are stateless firewalls for subnets. Option B is wrong because IAM does not control network traffic. Option C is wrong because CloudWatch monitors, not controls. Option E is wrong because KMS manages encryption keys.
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.
✓
Network ACLs
Why this is correct
NACLs are stateless firewalls applied at the subnet level.
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 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
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: Network ACLs — Options A and D are correct because security groups are stateful firewalls for instances, and NACLs are stateless firewalls for subnets. Option B is wrong because IAM does not control network traffic. Option C is wrong because CloudWatch monitors, not controls. Option E is wrong because KMS manages encryption keys.
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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