The most cost-effective way to provide internet access to private subnet instances is to launch a NAT instance in the public subnet and configure the private subnet route table to point 0.0.0.0/0 to that NAT instance. This works because a NAT instance, typically an Amazon Linux AMI with IP forwarding enabled, performs source network address translation (SNAT) for outbound traffic, allowing instances in the private subnet to reach the internet through the Internet Gateway while remaining unreachable from outside. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of cost optimization trade-offs between managed and self-managed network components; a common trap is confusing a bastion host’s SSH-only role with NAT functionality or assuming VPC endpoints can replace general internet access. Remember the mnemonic: “Bastion for bastards, NAT for packets” — bastion hosts handle administrative access, while NAT instances handle outbound packet translation.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. 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 wants to provide internet access to instances in a private subnet without using a NAT Gateway, due to cost constraints. They have a public subnet with a bastion host that has a public IP. They also have a VPC with an Internet Gateway. What is the most cost-effective way to allow outbound internet access for instances in the private subnet?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Launch a NAT instance in the public subnet and configure the private subnet route table to point 0.0.0.0/0 to the NAT instance.
Option B is correct. A NAT instance (Amazon Linux AMI with IP forwarding) in a public subnet can be used as a cheaper alternative to NAT Gateway, though it requires management. Option A is incorrect because bastion host is for SSH, not NAT. Option C is incorrect because VPC endpoints are for specific services, not general internet. Option D is incorrect because VPN is costly and not for internet access.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Launch a NAT instance in the public subnet and configure the private subnet route table to point 0.0.0.0/0 to the NAT instance.
Why this is correct
NAT instance is a cost-effective alternative to NAT Gateway.
Create a VPC endpoint for Amazon S3 and configure the private subnet to use it.
Why it's wrong here
VPC endpoint only provides access to specific AWS services, not the internet.
✗
Configure the bastion host as a NAT instance by enabling IP forwarding.
Why it's wrong here
Bastion host is for management, not for NAT; but could be used if configured, but usually not recommended.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Launch a NAT instance in the public subnet and configure the private subnet route table to point 0.0.0.0/0 to the NAT instance. — Option B is correct. A NAT instance (Amazon Linux AMI with IP forwarding) in a public subnet can be used as a cheaper alternative to NAT Gateway, though it requires management. Option A is incorrect because bastion host is for SSH, not NAT. Option C is incorrect because VPC endpoints are for specific services, not general internet. Option D is incorrect because VPN is costly and not for internet access.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
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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 needs to provide internet access to instances in a private subnet while preventing inbound connections from the internet. Which AWS service should be used?
easy
✓ A.NAT Gateway
B.Internet Gateway
C.AWS Direct Connect
D.VPC Endpoint
Why A: A NAT Gateway enables instances in a private subnet to initiate outbound IPv4 traffic to the internet (e.g., for software updates) while preventing any unsolicited inbound connections from the internet. It achieves this by translating the private source IP addresses of the instances to the NAT Gateway's Elastic IP address, and by default it does not allow inbound traffic unless an explicit inbound rule is added to the associated route table, which is not recommended for this use case.
Variation 2. A company needs to provide internet access to instances in a private subnet. The instances must not be directly accessible from the internet. Which AWS service should be used?
easy
A.VPC Peering
B.AWS Direct Connect
C.Internet Gateway
✓ D.NAT Gateway
Why D: A NAT Gateway enables instances in a private subnet to initiate outbound IPv4 traffic to the internet (e.g., for software updates) while preventing unsolicited inbound connections from the internet. It resides in a public subnet with an Elastic IP and uses source network address translation (SNAT) to translate the private IPs of the instances to the gateway's public IP, ensuring the instances remain inaccessible from the outside.
Variation 3. A company wants to provide internet access to instances in a private subnet using a NAT gateway. The NAT gateway is deployed in a public subnet with an Elastic IP. The private subnet route table has a default route pointing to the NAT gateway. However, instances in the private subnet cannot access the internet. What is the most likely cause?
easy
✓ A.The public subnet route table does not have a route to the internet gateway.
B.The security group assigned to the NAT gateway blocks outbound traffic.
C.The private subnet route table has a route to the internet gateway instead of the NAT gateway.
D.The NAT gateway does not have an Elastic IP associated.
Why A: The NAT gateway is deployed in a public subnet, but for it to route 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 (IGW). Without this route, the NAT gateway cannot forward traffic from the private subnet to the internet, even though the private subnet's route table correctly points to the NAT gateway. This is the most likely cause because the NAT gateway itself needs internet access via the IGW to translate and forward traffic.
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Question Discussion
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