The correct answer is to remove the public IP address from the instance and ensure the private subnet has a route to the NAT gateway. This is because a public IP address alone does not grant internet access; traffic must be routed through an Internet Gateway (IGW), and the private subnet’s route table deliberately lacks a default route to the IGW. Instead, the private subnet relies on a NAT gateway to translate outbound traffic from private IPs, so assigning a public IP to the instance creates a false expectation of direct connectivity while bypassing the intended NAT architecture. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of how routing tables and NAT gateways enforce network segmentation—a common trap is assuming a public IP overrides subnet routing. Remember the key principle: in a private subnet, internet access depends on the route table, not the instance’s IP assignment. A useful memory tip is “Public IP ≠ Public route; NAT is the private subnet’s gate.”
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 setting up a new VPC with public and private subnets. The VPC has an Internet Gateway attached. The public subnet's route table has a default route (0.0.0.0/0) pointing to the Internet Gateway. The private subnet's route table has a default route pointing to a NAT gateway. The engineer launches an EC2 instance in the private subnet and assigns it a public IP address. However, the instance cannot access the internet. What should the engineer do to resolve this issue?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Remove the public IP address from the instance and ensure the private subnet has a route to the NAT gateway.
Option C is correct. If an instance in a private subnet is assigned a public IP, it still cannot access the internet directly because the route table does not have a route to the Internet Gateway. The correct approach is to not assign a public IP and use a NAT gateway. Option A is wrong because the route table is correct for a private subnet. Option B is wrong because the security group is not the issue. Option D is wrong because the IGW is already attached.
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.
✗
Attach an Elastic IP to the instance.
Why it's wrong here
Even with an Elastic IP, the instance cannot reach the internet without a route to IGW.
✗
Update the private subnet's route table to point 0.0.0.0/0 to the Internet Gateway.
Why it's wrong here
This would make the subnet public, not private.
✓
Remove the public IP address from the instance and ensure the private subnet has a route to the NAT gateway.
Why this is correct
Private subnet instances should not have public IPs; they use NATgateway for internet access.
Allow outbound traffic in the instance's security group.
Why it's wrong here
Security groups allow all outbound traffic by default.
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 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: Remove the public IP address from the instance and ensure the private subnet has a route to the NAT gateway. — Option C is correct. If an instance in a private subnet is assigned a public IP, it still cannot access the internet directly because the route table does not have a route to the Internet Gateway. The correct approach is to not assign a public IP and use a NAT gateway. Option A is wrong because the route table is correct for a private subnet. Option B is wrong because the security group is not the issue. Option D is wrong because the IGW is already attached.
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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