ANS-C01 Network Implementation Practice Question
A company is deploying a VPC with public and private subnets in two Availability Zones. The public subnets are used for NAT gateways and an Application Load Balancer (ALB). The private subnets host EC2 instances running a web application. What is the most cost-effective and highly available configuration for internet access from the private instances?
⚠ Common exam trap
The ANS-C01 exam often tests the misconception that a single NAT gateway is sufficient for high availability, but the trap here is that a single NAT gateway is a single point of failure and incurs cross-AZ data transfer costs, making per-AZ NAT gateways the correct cost-effective and highly available design.
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
✓
Deploy a NAT gateway in each public subnet and configure private route tables with a default route to the NAT gateway in the same AZ.
Deploying a NAT gateway in each public subnet (one per Availability Zone) ensures that private instances in each AZ can route outbound traffic through a NAT gateway in the same AZ, providing both high availability (no single point of failure) and cost efficiency (no cross-AZ data transfer charges). This configuration uses the default route (0.0.0.0/0) in the private route tables pointing to the NAT gateway in the same AZ, which avoids the per-GB data transfer costs that would occur if traffic crossed AZs to reach a single NAT gateway.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Deploy a NAT gateway in each public subnet and configure private route tables with a default route to the NAT gateway in the same AZ.
Why this is correct
Provides high availability (each AZ independent) and uses managed service, cost-effective for moderate traffic.
- ✗
Create a single NAT gateway in one public subnet and route all private traffic to it.
Why it's wrong here
Single point of failure; if that AZ fails, private instances lose internet access.
- ✗
Attach an internet gateway to the VPC and add a default route to it in the private subnets.
Why it's wrong here
Private subnets need a NAT device, not direct internet gateway access.
- ✗
Launch a NAT instance in each public subnet and configure the private route tables.
Why it's wrong here
NAT instances require manual scaling and patching, less reliable than managed NAT gateways.
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Same concept, more angles
4 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 is deploying a VPC with public and private subnets in two Availability Zones. The public subnets host NAT gateways for outbound internet access from the private subnets. Which configuration ensures that EC2 instances in the private subnets can route traffic to the internet through the NAT gateways?
easy- A.Add a route in the public subnet route table pointing to the internet gateway.
- B.Attach an egress-only internet gateway to the private subnets.
- C.Create a VPC peering connection between the VPC and the NAT gateway.
- ✓ D.Add a route in the private subnet route table with destination 0.0.0.0/0 targeting the NAT gateway.
Why D: For EC2 instances in private subnets to reach the internet via a NAT gateway, the private subnet route table must include a default route (0.0.0.0/0) pointing to the NAT gateway as the next hop. The NAT gateway resides in a public subnet and translates the private IPs to its own Elastic IP, forwarding traffic to the internet gateway. Without this route, traffic from private instances has no path to the NAT gateway.
Variation 2. A company is deploying a VPC with public and private subnets in two Availability Zones. They need to ensure that instances in private subnets can access the internet for software updates while remaining unreachable from the internet. Which solution meets these requirements?
easy- A.Attach an internet gateway to the private subnets and configure route tables.
- ✓ B.Deploy a NAT Gateway in a public subnet and add a route to the NAT Gateway in the private subnet route tables.
- C.Use a transit gateway to connect the VPC to the internet.
- D.Establish a VPN connection to an on-premises network and route traffic through it.
Why B: A NAT Gateway deployed in a public subnet provides outbound-only internet access for instances in private subnets. The private subnet route table directs 0.0.0.0/0 traffic to the NAT Gateway, which translates the private IPs to the NAT Gateway's Elastic IP, allowing internet access while preventing any unsolicited inbound connections from the internet.
Variation 3. A company is deploying a VPC with public and private subnets in two Availability Zones. The workloads in the private subnets need to access the internet for software updates. What is the MOST secure way to provide this internet access?
easy- ✓ A.Deploy a NAT gateway in a public subnet and update the private subnets' route table to point default traffic to the NAT gateway.
- B.Set up a VPN connection to an on-premises network with internet access.
- C.Use a transit gateway with a VPC attachment and route internet traffic through a centralized inspection VPC.
- D.Attach an internet gateway to the VPC and route the private subnets' traffic to it.
Why A: A NAT gateway in a public subnet allows instances in private subnets to initiate outbound traffic to the internet (e.g., for software updates) while preventing any unsolicited inbound traffic from the internet. This is the most secure and AWS-recommended method because it uses Source Network Address Translation (SNAT) to mask private IPs behind the NAT gateway's Elastic IP, and it requires only a route table update in the private subnets to direct default traffic (0.0.0.0/0) to the NAT gateway.
Variation 4. A company is deploying a VPC with public and private subnets in two Availability Zones. The private subnets need outbound internet access for software updates but must not be reachable from the internet. Which AWS service should be used to achieve this?
medium- A.VPC Peering
- B.Virtual Private Gateway
- ✓ C.NAT Gateway
- D.Internet Gateway
Why C: A NAT Gateway enables instances in private subnets to initiate outbound IPv4 traffic to the internet (e.g., for software updates) while preventing the internet from initiating connections back to those instances. It resides in a public subnet with an Elastic IP and uses source network address translation (SNAT) to translate private source IPs to the gateway's public IP, ensuring the private instances remain unreachable from external hosts.
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.