- A
Configure the security group of the NAT Gateway to allow inbound traffic from the private subnet.
Why wrong: NAT Gateway security groups are not used for traffic routing; it's a managed service.
- B
Add a route in the private subnet route table to the NAT Gateway's private IP.
Why wrong: The route already points to the NAT Gateway's private IP correctly.
- C
Add a route in the public subnet route table to the Internet Gateway.
Why wrong: Incorrect. While a route to the Internet Gateway is necessary for the public subnet, the question asks for additional configuration for the NAT Gateway itself. The public subnet likely already has such a route; the key missing element is the Elastic IP address.
- D
Assign an Elastic IP address to the NAT Gateway.
Correct. An Elastic IP address is a required component for a NAT Gateway in a public subnet. It provides a public IP for internet communication.
Troubleshooting NAT Gateway Missing Public Subnet 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. A key principle to apply: nAT Gateway Elastic IP Requirement. 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 a public subnet and a private subnet. An EC2 instance in the private subnet needs to download patches from the internet. The instance is associated with a security group that allows outbound HTTPS (443) traffic. The route table for the private subnet has a default route pointing to a NAT Gateway in the public subnet. Which additional configuration is required to ensure the NAT Gateway can route the traffic?
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
Assign an Elastic IP address to the NAT Gateway.
A NAT Gateway must have an Elastic IP address assigned to it to communicate with the internet. Without an Elastic IP, the NAT Gateway cannot send traffic to the internet or receive return traffic. The private subnet route table correctly points to the NAT Gateway, and the security group on the instance allows outbound HTTPS. The missing configuration is the Elastic IP address, which is required for the NAT Gateway to function.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configure the security group of the NAT Gateway to allow inbound traffic from the private subnet.
- ✗
Add a route in the private subnet route table to the NAT Gateway's private IP.
- ✗
Add a route in the public subnet route table to the Internet Gateway.
- ✓
Assign an Elastic IP address to the NAT Gateway.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The ANS-C01 exam often tests the misconception that a NAT Gateway automatically has internet access if it is in a public subnet, but the trap here is that the public subnet's route table must explicitly have a default route to the Internet Gateway for the NAT Gateway to route traffic to the internet.
Detailed technical explanation
How to think about this question
The NAT Gateway acts as a bridge between the private subnet and the internet: it receives traffic from the private subnet, performs source network address translation (SNAT) using its Elastic IP, and forwards packets to the IGW. The IGW must be the target of the default route in the public subnet's route table for the NAT Gateway to reach the internet. A common subtlety is that the NAT Gateway's Elastic IP is used for SNAT, but the routing path still depends on the public subnet's route table having a 0.0.0.0/0 route to the IGW; without it, packets from the NAT Gateway are dropped.
KKey Concepts to Remember
- NAT Gateway Elastic IP Requirement
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
NAT Gateway Elastic IP Requirement
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
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Implementation — This question tests Network Implementation — NAT Gateway Elastic IP Requirement.
What is the correct answer to this question?
The correct answer is: Assign an Elastic IP address to the NAT Gateway. — A NAT Gateway must have an Elastic IP address assigned to it to communicate with the internet. Without an Elastic IP, the NAT Gateway cannot send traffic to the internet or receive return traffic. The private subnet route table correctly points to the NAT Gateway, and the security group on the instance allows outbound HTTPS. The missing configuration is the Elastic IP address, which is required for the NAT Gateway to function.
What should I do if I get this ANS-C01 question wrong?
Review nAT Gateway Elastic IP Requirement, then practise related ANS-C01 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
NAT Gateway Elastic IP Requirement
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Same concept, more angles
2 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. An EC2 instance in the private subnet needs to access the internet to download software updates. The company has a NAT gateway in the public subnet. The route table for the private subnet has a default route (0.0.0.0/0) pointing to the NAT gateway. However, the EC2 instance cannot access the internet. The network engineer verifies that the NAT gateway has an Elastic IP, the security group for the EC2 instance allows outbound HTTPS traffic, and the network ACL for the private subnet allows inbound and outbound ephemeral ports. What is the most likely cause of the issue?
hard- A.The security group attached to the NAT gateway does not allow outbound traffic.
- ✓ B.The route table for the public subnet does not have a route to an internet gateway.
- C.The route table for the private subnet does not have a route to the NAT gateway.
- D.The NAT gateway does not support UDP traffic.
Why B: The NAT gateway resides in the public subnet, which must have a route table with a default route (0.0.0.0/0) pointing to an internet gateway (IGW) for the NAT gateway to forward traffic to the internet. Without this route, the NAT gateway cannot reach the IGW, so traffic from the private subnet EC2 instance fails even though the private subnet route is correctly configured. The security group and network ACL settings are irrelevant if the NAT gateway itself lacks internet connectivity.
Variation 2. A company has a VPC with a public subnet and a private subnet. An EC2 instance in the private subnet needs to download patches from the internet. The company has a NAT gateway in the public subnet. The route table for the private subnet has a route 0.0.0.0/0 pointing to the NAT gateway. However, the EC2 instance cannot reach the internet. Which additional configuration is needed?
hard- A.Assign a public IP address to the EC2 instance.
- B.Attach a security group to the NAT gateway allowing outbound traffic.
- C.Add a route to the NAT gateway in the private subnet's route table for destination 0.0.0.0/0.
- ✓ D.Ensure the public subnet's route table has a route 0.0.0.0/0 pointing to an internet gateway.
Why D: The NAT gateway resides in the public subnet, but it still needs a route to the internet. The public subnet's route table must have a default route (0.0.0.0/0) pointing to an internet gateway (IGW) so that traffic from the NAT gateway can be forwarded to the internet. Without this route, the NAT gateway cannot send traffic beyond the VPC, even though the private subnet's route table correctly points to the NAT gateway.
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Last reviewed: Jul 4, 2026
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