Why Your NAT Gateway Needs an Elastic IP Address
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?
⚠ Common exam trap
Many exam-takers confuse a NAT Gateway with a NAT instance, thinking the latter is acceptable, but the question explicitly requires minimizing operational overhead, which disqualifies the self-managed NAT instance in favor of the fully managed NAT Gateway.
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
✓
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.
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Place the EC2 instances in a public subnet and configure security group inbound rules to block all traffic.
Why it's wrong here
Instances in a public subnet have direct internet access but are still reachable from the internet if security groups are misconfigured; this does not guarantee the required security posture and does not fully prevent inbound traffic.
- ✓
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.
Why this is correct
A NAT Gateway deployed in a public subnet with an Elastic IP provides secure outbound-only internet connectivity for private-instance traffic. The private subnet route table's 0.0.0.0/0 entry points to the NAT gateway, which translates source IPs and discards unsolicited inbound connections. Because AWS fully manages the gateway, it auto-scales and requires no patching, minimizing operational overhead. This satisfies both security and administrative efficiency.
- ✗
Launch a NAT instance in a public subnet with an Elastic IP address and configure route tables accordingly.
Why it's wrong here
While a NAT instance in a public subnet with an Elastic IP can also enable outbound access, it is essentially an EC2 instance you must configure and manage yourself. You are responsible for OS patching, disabling source/destination-check, tuning the security group, and scaling if traffic grows. It also becomes a single point of failure unless you build custom high-availability mechanisms. This operational burden directly contradicts the goal of minimizing overhead.
- ✗
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 it's wrong here
Attaching an Internet Gateway and routing 0.0.0.0/0 from the private subnet directly to it is a fundamental VPC design error. An Internet Gateway only works for instances with public IPs; it does not perform source-NAT, so private instances would either have no connectivity or, if assigned public IPs, would be directly exposed to inbound internet traffic. This defeats the purpose of a private subnet and creates a security vulnerability, unlike a managed NAT Gateway that provides one-way connectivity.
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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.