Quick Answer
The answer is security groups, as they provide the stateful firewall within a VPC that automatically allows return traffic. Security groups operate at the elastic network interface level, tracking connection state so that if you permit inbound traffic, the corresponding outbound response is automatically allowed without needing a separate rule. On the AWS Certified Security Specialty SCS-C02 exam, this question tests your ability to distinguish between stateful and stateless filtering mechanisms, with network ACLs being the primary trap because they are stateless and require explicit rules for both directions. A common memory tip is to think of security groups as a “smart bouncer” who remembers who came in and lets them leave, while network ACLs are a “forgetful guard” who checks every packet both ways. For VPC traffic control between subnets, always default to security groups when stateful behavior is required.
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 VPC with multiple subnets. The security team wants to control traffic between subnets using a stateful firewall that can automatically allow return traffic. Which AWS service should be used?
Network ACLs
Why wrong: Network ACLs are stateless.
AWS Firewall Manager
Why wrong: Firewall Manager manages firewall rules centrally.
AWS WAF
Why wrong: WAF is for web application firewall.
Security groups
Security groups are stateful and can be used for subnet-level traffic control.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Security groups
Option B is correct because security groups are stateful and can be used to control traffic between subnets when attached to ENIs. Option A is wrong because network ACLs are stateless. Option C is wrong because AWS WAF is for web traffic at the application layer. Option D is wrong because AWS Firewall Manager is a policy management service, not a firewall itself.
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.
Network ACLs
Why it's wrong here
Network ACLs are stateless.
AWS Firewall Manager
Why it's wrong here
Firewall Manager manages firewall rules centrally.
AWS WAF
Why it's wrong here
WAF is for web application firewall.
Security groups
Why this is correct
Security groups are stateful and can be used for subnet-level traffic control.
Related concept
CIDR notation defines the prefix length.
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 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
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: Security groups — Option B is correct because security groups are stateful and can be used to control traffic between subnets when attached to ENIs. Option A is wrong because network ACLs are stateless. Option C is wrong because AWS WAF is for web traffic at the application layer. Option D is wrong because AWS Firewall Manager is a policy management service, not a firewall itself.
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
This SCS-C02 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SCS-C02 exam.
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