The answer is to configure the private subnet route table with no route to an Internet Gateway or NAT Gateway, and to use a security group that allows inbound traffic only from the public subnet. This works because a private subnet with no default route to 0.0.0.0/0 cannot initiate outbound connections to the internet, while a properly scoped security group permits responses to inbound requests from within the VPC. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of the fundamental difference between stateful security groups and stateless network ACLs, as well as the specific roles of NAT and Internet Gateways. A common trap is assuming a NAT Gateway is required for any internet traffic; remember that NAT Gateway enables outbound-only access, so it is explicitly wrong when the requirement is no outbound initiation. Memory tip: "No route, no outbound—security group lets responses inbound."
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 security engineer is configuring a VPC with public and private subnets. The engineer wants to ensure that the private subnet instances cannot initiate outbound connections to the internet but can receive responses from the internet if initiated from within the VPC. Which TWO configurations should be used?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure the private subnet security group to allow inbound traffic from the public subnet's security group.
Options A and D are correct. A NATGateway allows outbound internet but not inbound; however the question says cannot initiate outbound. So a NAT Gateway is not needed. The correct approach is to not have a route to the internet for private subnet, and use a security group that allows inbound traffic only from the public subnet. Option B is wrong because Internet Gateway is for public subnets. Option C is wrong because it allows outbound. Option E is wrong because Network ACLs are stateless and would need rules for both directions.
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.
✓
Configure the private subnet security group to allow inbound traffic from the public subnet's security group.
Attach an Internet Gateway to the VPC and add a route to it in the private subnet route table.
Why it's wrong here
Allows outbound internet.
✗
Attach a NAT Gateway to the private subnet and add a route to it.
Why it's wrong here
Allows outbound internet.
✗
Configure the private subnet Network ACL to allow outbound traffic to 0.0.0.0/0.
Why it's wrong here
Allows outbound internet.
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: Configure the private subnet security group to allow inbound traffic from the public subnet's security group. — Options A and D are correct. A NAT Gateway allows outbound internet but not inbound; however the question says cannot initiate outbound. So a NAT Gateway is not needed. The correct approach is to not have a route to the internet for private subnet, and use a security group that allows inbound traffic only from the public subnet. Option B is wrong because Internet Gateway is for public subnets. Option C is wrong because it allows outbound. Option E is wrong because Network ACLs are stateless and would need rules for both directions.
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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