The answer is that the NAT gateway is not in a public subnet with an internet gateway. A NAT gateway must reside in a public subnet that has a direct route to an internet gateway (IGW) in its route table; if it is placed in a private subnet, it has no path to the internet and cannot forward traffic from the EC2 instance. This scenario tests your understanding of how NAT gateways function as bridge devices—they require an IGW to translate private IPs, but the IGW itself must be attached to the NAT gateway’s subnet, not the instance’s subnet. On the AWS Certified Solutions Architect Professional SAP-C02 exam, this is a classic trap: candidates see a default route pointing to a NAT gateway and assume it works, but they overlook the gateway’s subnet placement. A common memory tip is “NAT needs a public home”—the gateway itself must live in a subnet with an IGW route, while the private instances simply point their default route to the NAT gateway’s IP.
SAP-C02 Continuous Improvement for Existing Solutions Practice Question
This SAP-C02 practice question tests your understanding of continuous improvement for existing solutions. 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.
Network Topology
Refer to the exhibit. An EC2 instance in subnet-11111 (10.0.1.0/24) cannot access the internet. The route table for the subnet is shown. What is the MOST likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The NAT gateway is not in a public subnet with an internet gateway.
Option C is correct. The route table has a route to a NAT gateway in the same subnet, but the NAT gateway must be in a public subnet with an internet gateway. If the NAT gateway is in a private subnet, it cannot route to the internet. Option A is wrong because there is a default route. Option B is wrong because the local route is present. Option D is wrong because the route table is associated with the subnet.
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.
✗
The route table does not have a default route (0.0.0.0/0).
Why it's wrong here
The route table shows a 0.0.0.0/0 route.
✗
The VPC does not have a local route.
Why it's wrong here
The local route is present.
✗
The route table is not associated with the subnet.
Why it's wrong here
The association shows the route table is associated with subnet-11111.
✓
The NAT gateway is not in a public subnet with an internet gateway.
Why this is correct
NATgateway requires a public subnet and internet gateway to function.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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.
Trap categories for this question
Command / output trap
The route table shows a 0.0.0.0/0 route.
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 SAP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Continuous Improvement for Existing Solutions — This question tests Continuous Improvement for Existing Solutions — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The NAT gateway is not in a public subnet with an internet gateway. — Option C is correct. The route table has a route to a NAT gateway in the same subnet, but the NAT gateway must be in a public subnet with an internet gateway. If the NAT gateway is in a private subnet, it cannot route to the internet. Option A is wrong because there is a default route. Option B is wrong because the local route is present. Option D is wrong because the route table is associated with the subnet.
What should I do if I get this SAP-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 SAP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company deploys the above CloudFormation template. After deployment, an EC2 instance launched in mySubnet can access the internet. However, the instance cannot receive inbound traffic from the internet. What is the MOST likely reason?
medium
A.The subnet does not have auto-assign public IP enabled.
✓ B.No security group or network ACL allows inbound traffic.
C.The VPC does not have an internet gateway attached.
D.The route table does not have a route to the internet gateway.
Why B: Option B is correct because the template does not configure a security group or network ACL to allow inbound traffic. The route table and internet gateway are correctly set up for outbound traffic. Option A is wrong because the subnet has MapPublicIpOnLaunch: true. C is wrong because the route exists. D is wrong because the VPC is not missing an internet gateway.
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Question Discussion
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