- A
The NAT Gateway is deployed in the private subnet instead of the public subnet.
NAT Gateway must be in a public subnet with a route to IGW to function.
- B
The security group attached to the NAT Gateway blocks outbound traffic.
Why wrong: NAT Gateway does not have security groups.
- C
The private subnet route table does not have a default route (0.0.0.0/0) to the NAT Gateway.
Why wrong: The stem states the route is present.
- D
The NAT Gateway is in a private subnet and has no route to the internet gateway.
Why wrong: The issue is the NAT Gateway placement, not its route.
Question 354 of 1,705
Why Deploying NAT Gateway in Private Subnet Breaks Internet Access
This ANS-C01 practice question tests your understanding of network management and operations. 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 company uses AWS CloudFormation to deploy a VPC with public and private subnets. The template includes an Internet Gateway and a NAT Gateway. After deployment, instances in the private subnet cannot access the internet. The network engineer checks the route tables and finds that the private subnet route table has a default route pointing to the NAT Gateway. What is the most likely cause of the issue?
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 choices
Why each option matters
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 deployed in the private subnet instead of the public subnet.
Option A is correct because the NAT Gateway must be deployed in a public subnet with a route to the Internet Gateway. If it is in a private subnet, it cannot reach the internet, so instances in the private subnet cannot use it for outbound traffic. Option B is incorrect because security groups by default allow outbound traffic. Option C is incorrect because the question states the private subnet route table has a default route pointing to the NAT Gateway. Option D is incorrect because while the NAT Gateway being in a private subnet and lacking a route to the IGW is a related issue, the most direct cause is that the NAT Gateway itself is in the wrong subnet, as stated in option A.
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 NAT Gateway is deployed in the private subnet instead of the public subnet.
- ✗
The security group attached to the NAT Gateway blocks outbound traffic.
- ✗
The private subnet route table does not have a default route (0.0.0.0/0) to the NAT Gateway.
Why it's wrong here
The stem states the route is present.
- ✗
The NAT Gateway is in a private subnet and has no route to the internet gateway.
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.
Visual reference
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 ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The NAT Gateway is deployed in the private subnet instead of the public subnet. — Option A is correct because the NAT Gateway must be deployed in a public subnet with a route to the Internet Gateway. If it is in a private subnet, it cannot reach the internet, so instances in the private subnet cannot use it for outbound traffic. Option B is incorrect because security groups by default allow outbound traffic. Option C is incorrect because the question states the private subnet route table has a default route pointing to the NAT Gateway. Option D is incorrect because while the NAT Gateway being in a private subnet and lacking a route to the IGW is a related issue, the most direct cause is that the NAT Gateway itself is in the wrong subnet, as stated in option A.
What should I do if I get this ANS-C01 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 ANS-C01 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.
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Last reviewed: Jun 20, 2026
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