ANS-C01 Network Implementation Practice Question
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.
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
Why it's wrong here
It is already pointed 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
Why this is correct
The NAT gateway needs internet access itself to forward traffic.
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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: 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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.