The answer is to attach an Internet Gateway to the VPC and configure a NAT Gateway in a public subnet with an Elastic IP. This works because the NAT Gateway, residing in a public subnet with an Internet Gateway, translates private instance traffic to a single static IP address for outbound requests, while the private subnet’s route table directs default traffic (0.0.0.0/0) to the NAT Gateway. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of how to maintain a consistent outbound static IP for third-party whitelisting without exposing private instances to inbound connections. A common trap is confusing a NAT Instance or a VPC endpoint for this purpose, but remember: only a NAT Gateway with an Elastic IP provides a fixed, scalable source IP for outbound traffic. Memory tip: “NAT needs an IGW to get out, and an EIP to stay fixed.”
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 security engineer is designing a VPC with public and private subnets. The company requires that all outbound traffic from private subnets to the internet must go through a single IP address for whitelisting by third-party services. Which TWO actions should the engineer take?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Deploy a NAT Gateway in a public subnet and assign an Elastic IP.
A NAT Gateway in a public subnet with an Elastic IP allows private instances to access the internet using a single IP. Route tables for private subnets must point default route (0.0.0.0/0) to the NAT Gateway.
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.
✗
Use a VPC Peering connection to a central VPC with an Internet Gateway.
Why it's wrong here
VPC Peering does not provide internet access; a NAT or IGW is still needed.
✗
Deploy a NAT Gateway in a private subnet.
Why it's wrong here
NATGateway must be in a public subnet to have internet access.
✗
Configure the private subnet route table with a default route to an Internet Gateway.
Why it's wrong here
Private subnets cannot route directly to Internet Gateway; they must go through NAT.
✓
Deploy a NAT Gateway in a public subnet and assign an Elastic IP.
Why this is correct
Correct placement and IP assignment for outbound traffic.
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: Deploy a NAT Gateway in a public subnet and assign an Elastic IP. — A NAT Gateway in a public subnet with an Elastic IP allows private instances to access the internet using a single IP. Route tables for private subnets must point default route (0.0.0.0/0) to the NAT Gateway.
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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