- A
The network ACL for the private subnet allows outbound traffic
Why wrong: Default NACL allows all traffic.
- B
The private subnet route table is missing a route to the NAT gateway
Why wrong: It is already pointed to the NAT gateway.
- C
The security group for the instances allows outbound traffic
Why wrong: Default security group allows all outbound traffic.
- D
The route table for the public subnet where the NAT gateway resides does not have a default route to an internet gateway
The NAT gateway needs internet access itself to forward traffic.
Question 1,428 of 1,705
NAT Gateway Internet Access Troubleshooting: Missing Route to Internet Gateway
This ANS-C01 practice question tests your understanding of network implementation. 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 has a VPC with public and private subnets. They have a NAT gateway in a public subnet. They want to provide internet access to instances in private subnets. The NAT gateway is configured with an Elastic IP. The private instances still cannot access the internet. The route table for the private subnets has a default route (0.0.0.0/0) pointing to the NAT gateway. What is missing?
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 route table for the public subnet where the NAT gateway resides does not have a default route to an internet gateway
The private instances can't access the internet because the NAT gateway itself is in a public subnet that lacks a route to an internet gateway. The route table for the public subnet where the NAT gateway resides must have a default route (0.0.0.0/0) pointing to an internet gateway for the NAT gateway to reach the internet. Without this, the NAT gateway cannot forward traffic from the private instances to the internet. Option A is not the best answer because network ACLs are stateless and by default allow outbound traffic, but they could be misconfigured; however, the most common missing piece is the public subnet's route table. Option B is incorrect because the private subnet route table already has the default route to the NAT gateway. Option C is incorrect because security groups are stateful and automatically allow outbound traffic if inbound is allowed, so they are not blocking.
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 network ACL for the private subnet allows outbound traffic
Why it's wrong here
Default NACL allows all traffic.
- ✗
The private subnet route table is missing a route to the NAT gateway
- ✗
The security group for the instances allows outbound traffic
Why it's wrong here
Default security group allows all outbound traffic.
- ✓
The route table for the public subnet where the NAT gateway resides does not have a default route to an 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 Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The route table for the public subnet where the NAT gateway resides does not have a default route to an internet gateway — The private instances can't access the internet because the NAT gateway itself is in a public subnet that lacks a route to an internet gateway. The route table for the public subnet where the NAT gateway resides must have a default route (0.0.0.0/0) pointing to an internet gateway for the NAT gateway to reach the internet. Without this, the NAT gateway cannot forward traffic from the private instances to the internet. Option A is not the best answer because network ACLs are stateless and by default allow outbound traffic, but they could be misconfigured; however, the most common missing piece is the public subnet's route table. Option B is incorrect because the private subnet route table already has the default route to the NAT gateway. Option C is incorrect because security groups are stateful and automatically allow outbound traffic if inbound is allowed, so they are not blocking.
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Same concept, more angles
1 more ways this is tested on ANS-C01
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 has a VPC with a public subnet and a private subnet. They have a NAT Gateway in the public subnet. They also have an EC2 instance in the private subnet that needs to access the internet. The route table for the private subnet has a default route (0.0.0.0/0) pointing to the NAT Gateway. The security group for the EC2 instance allows outbound HTTPS traffic. The network ACL for the private subnet allows inbound and outbound ephemeral ports. However, the EC2 instance cannot reach the internet. The network engineer checks the NAT Gateway and sees that it has an Elastic IP attached. The engineer also checks the route table for the public subnet and finds no route to the internet. What should the engineer do to fix the issue?
medium- A.Deploy a second NAT Gateway in the private subnet.
- ✓ B.Add a route in the public subnet route table to 0.0.0.0/0 via the internet gateway.
- C.Add a route in the private subnet route table to 0.0.0.0/0 via the internet gateway.
- D.Attach a second Elastic IP to the NAT Gateway.
Why B: Option B is correct because the NAT Gateway resides in the public subnet, and for the NAT Gateway to forward traffic to the internet, the public subnet's route table must have a default route (0.0.0.0/0) pointing to an internet gateway. Without that route, the NAT Gateway cannot send traffic to the internet, even though the private subnet's route points to the NAT Gateway. Options A, C, and D are incorrect: A is unnecessary because a NAT Gateway in the private subnet would not have direct internet access; C would bypass the NAT Gateway and require a route to the internet gateway directly from the private subnet, which is not the standard design; D is wrong because adding another Elastic IP does not solve the missing route issue.
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Last reviewed: Jun 20, 2026
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