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Network Design →mediumMultiple ChoiceObjective-mapped

Quick Answer

The correct design is to attach a NAT Gateway to the public subnet and add a route to the NAT Gateway in the private subnet’s route table. This works because the NAT Gateway, residing in a public subnet with an Elastic IP, translates outbound traffic from private instances to its own public IP, allowing them to download patches while the stateful firewall drops any unsolicited inbound traffic from the internet. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how to enable outbound internet access from a private subnet without exposing instances to direct inbound connections—a core concept for secure VPC design. A common trap is confusing a NAT Gateway with an Internet Gateway; remember that an Internet Gateway alone requires public IPs on instances, which would break the “no direct inbound” requirement. Memory tip: think of the NAT Gateway as a one-way valve—outbound yes, inbound no.

ANS-C01 Network Design Practice Question

This ANS-C01 practice question tests your understanding of network design. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 has a VPC with public and private subnets. An Amazon EC2 instance in a private subnet needs to download patches from the internet. The company wants to ensure that the instance cannot be directly initiated from the internet. Which design should be used?

Question 1mediummultiple choice

Read the full NAT/PAT explanation →

A
Attach a NAT Gateway to the public subnet and add a route to the NAT Gateway in the private subnet's route table
NAT Gateway enables outbound internet access from private subnets and does not allow inbound connections.
B
Launch a NAT instance in the public subnet and add a route in the private subnet's route table
Why wrong: A NAT instance can work but is less reliable and more effort to manage than a NAT Gateway.
C
Use a VPC endpoint for Amazon S3 and route patch traffic through it
Why wrong: VPC endpoints only provide access to AWS services, not to general internet resources.
D
Add a route to the internet gateway in the private subnet's route table
Why wrong: Instances in private subnets do not have public IPs, so traffic to the internet gateway would fail.

Full breakdown with real-world context →

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 the public subnet and add a route to the NAT Gateway in the private subnet's route table

Option D is correct because a NAT Gateway in a public subnet allows outbound internet access from private subnets while preventing inbound connections from the internet. Option A is wrong because an Internet Gateway alone does not provide outbound-only access; it requires public IPs. Option B is wrong because a NAT Instance also works but is less managed; however, the question asks for a design, and NAT Gateway is the recommended AWS managed service. Option C is wrong because a VPC endpoint for S3 is for accessing S3, not general internet patches.

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.

✓
Attach a NAT Gateway to the public subnet and add a route to the NAT Gateway in the private subnet's route table
Why this is correct
NAT Gateway enables outbound internet access from private subnets and does not allow inbound connections.
Related concept
CIDR notation defines the prefix length.
✗
Launch a NAT instance in the public subnet and add a route in the private subnet's route table
Why it's wrong here
A NAT instance can work but is less reliable and more effort to manage than a NAT Gateway.
✗
Use a VPC endpoint for Amazon S3 and route patch traffic through it
Why it's wrong here
VPC endpoints only provide access to AWS services, not to general internet resources.
✗
Add a route to the internet gateway in the private subnet's route table
Why it's wrong here
Instances in private subnets do not have public IPs, so traffic to the internet gateway would fail.

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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.

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.

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FAQ

Questions learners often ask

What does this ANS-C01 question test?

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: Attach a NAT Gateway to the public subnet and add a route to the NAT Gateway in the private subnet's route table — Option D is correct because a NAT Gateway in a public subnet allows outbound internet access from private subnets while preventing inbound connections from the internet. Option A is wrong because an Internet Gateway alone does not provide outbound-only access; it requires public IPs. Option B is wrong because a NAT Instance also works but is less managed; however, the question asks for a design, and NAT Gateway is the recommended AWS managed service. Option C is wrong because a VPC endpoint for S3 is for accessing S3, not general internet patches.

What should I do if I get this ANS-C01 question wrong?

What is the key concept behind this question?

CIDR notation defines the prefix length.

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