Why Your NAT Gateway Needs an Elastic IP Address
A SysOps Administrator manages a VPC with public and private subnets. The private subnets need to access the internet for software updates. The Administrator creates a NAT Gateway in a public subnet and updates the private subnet route table to point 0.0.0.0/0 to the NAT Gateway. However, instances in the private subnet still cannot reach the internet. What is the MOST likely reason?
Quick Answer
The answer is that the NAT Gateway lacks an Elastic IP address, which is a mandatory requirement for it to function. A NAT Gateway resides in a public subnet and must have an Elastic IP assigned to it so that the Internet Gateway can route return traffic back to the gateway; without this public IP, the NAT Gateway cannot translate private instance addresses for outbound internet access. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of the NAT Gateway Elastic IP requirement and how it differs from other connectivity components—a common trap is assuming a security group or route table misconfiguration is the culprit, but NAT Gateways do not use security groups. Remember the memory tip: “No EIP, no internet” for a NAT Gateway, and always verify that the Elastic IP is attached in the console before troubleshooting routes.
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 does not have an Elastic IP address assigned.
A NAT Gateway must have an Elastic IP (EIP) address assigned to it in order to function as an internet gateway for private subnets. Without an EIP, the NAT Gateway cannot communicate with the internet. Option A is unlikely because security groups allow outbound traffic by default. Option B is incorrect because the internet gateway attaches to the VPC, not to the NAT Gateway. Option D is incorrect because NAT Gateways do not have security groups; they use network ACLs at the subnet level, but the issue here is the missing EIP.
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 security group on the private instances blocks outbound HTTPS traffic.
Why it's wrong here
By default, security groups allow all outbound traffic; unless explicitly removed, outbound is allowed.
- ✗
The internet gateway is not attached to the NAT Gateway.
Why it's wrong here
The internet gateway is attached to the VPC, not to individual resources like NAT Gateways.
- ✓
The NAT Gateway does not have an Elastic IP address assigned.
Why this is correct
A NAT Gateway requires an Elastic IP to communicate with the internet; without one, it cannot route traffic.
- ✗
The NAT Gateway's security group is blocking inbound traffic from the private subnet.
Why it's wrong here
NAT Gateways do not have security groups; they use network ACLs and route tables.
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Same concept, more angles
1 more way this is tested on SOA-C02
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 an Application Load Balancer (ALB) that routes traffic to Amazon EC2 instances in private subnets of a VPC. The SysOps administrator needs to ensure that the EC2 instances can download software updates from the internet, but they must not be directly accessible from the internet. The solution should minimize operational overhead. Which solution should the administrator implement?
medium- A.Place the EC2 instances in a public subnet and configure security group inbound rules to block all traffic.
- ✓ B.Attach a NAT Gateway to a public subnet and configure the private subnet route table to send 0.0.0.0/0 traffic to the NAT Gateway.
- C.Launch a NAT instance in a public subnet with an Elastic IP address and configure route tables accordingly.
- D.Attach an Internet Gateway to the VPC and add a route to the private subnet route table pointing 0.0.0.0/0 to the Internet Gateway.
Why B: A NAT Gateway (option B) allows EC2 instances in private subnets to initiate outbound connections to the internet (e.g., for software updates) while preventing any unsolicited inbound connections from the internet. It is a fully managed AWS service that automatically scales and requires no patching, minimizing operational overhead compared to a NAT instance. The private subnet route table directs 0.0.0.0/0 traffic to the NAT Gateway, which is placed in a public subnet with an Elastic IP address to enable internet access.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This SOA-C02 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 SOA-C02 exam.