The answer is that the NAT Gateway is deployed in a private subnet. For a NAT Gateway to provide internet access to instances in a private subnet, it must be placed in a public subnet that has a route to an Internet Gateway. Without that public-facing placement, the NAT Gateway cannot translate private IP addresses to a public one, and the default route of 0.0.0.0/0 in the private subnet’s route table becomes a dead end. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this question tests your understanding of VPC routing fundamentals and the critical distinction between public and private subnet configurations. A common trap is assuming that simply pointing a route to the NAT Gateway is enough, but the gateway itself must be internet-reachable. Remember the memory tip: “NATs need a public spot to connect the private lot.”
SOA-C02 Networking and Content Delivery Practice Question
This SOA-C02 practice question tests your understanding of networking and content delivery. 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. The public subnet has a NAT Gateway. The private subnet has an EC2 instance that needs to download patches from the internet. The route table for the private subnet has a default route (0.0.0.0/0) pointing to the NAT Gateway. However, the instance cannot reach the internet. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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 is deployed in a private subnet.
Option D is correct because the NATGateway must be in a public subnet with an Internet Gateway route. If it's in a private subnet, it won't work. Option A is wrong because security groups control inbound/outbound traffic but the default outbound allows all; the issue is routing. Option B is wrong because NACLs are stateless but the default allows outbound. Option C is wrong because the NAT Gateway does not need an Elastic IP if it's in a public subnet, but it's not required for connectivity.
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.
✗
The network ACL for the private subnet blocks outbound HTTP traffic.
Why it's wrong here
Incorrect because the default NACL allows all outbound traffic.
✗
The security group of the EC2 instance blocks outbound traffic.
Why it's wrong here
Incorrect because the default security group allows all outbound traffic.
✓
The NAT Gateway is deployed in a private subnet.
Why this is correct
Correct because a NATGateway must be in a public subnet to route traffic to the internet.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The NAT Gateway does not have an Elastic IP address.
Why it's wrong here
Incorrect because NAT Gateways are automatically assigned public IPs; Elastic IP is optional.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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 SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Networking and Content Delivery — This question tests Networking and Content Delivery — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The NAT Gateway is deployed in a private subnet. — Option D is correct because the NAT Gateway must be in a public subnet with an Internet Gateway route. If it's in a private subnet, it won't work. Option A is wrong because security groups control inbound/outbound traffic but the default outbound allows all; the issue is routing. Option B is wrong because NACLs are stateless but the default allows outbound. Option C is wrong because the NAT Gateway does not need an Elastic IP if it's in a public subnet, but it's not required for connectivity.
What should I do if I get this SOA-C02 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 SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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 has a VPC with a public subnet and a private subnet. An EC2 instance in the private subnet needs to download patches from the internet. The company has a NAT gateway in the public subnet. Which of the following route table configurations is required for the private subnet to enable internet access through the NAT gateway?
hard
A.Add a route to 0.0.0.0/0 pointing to the internet gateway in the private subnet route table
✓ B.Add a route to 0.0.0.0/0 pointing to the NAT gateway in the private subnet route table
C.Add a route to 0.0.0.0/0 pointing to the NAT gateway in the public subnet route table
D.Add a route to the NAT gateway's private IP in the private subnet route table
Why B: Option C is correct because the private subnet's route table must have a default route (0.0.0.0/0) pointing to the NAT gateway. Option A is wrong because a route to the internet gateway would bypass the NAT gateway. Option B is wrong because the NAT gateway is in the public subnet. Option D is wrong because the private subnet does not need a direct route to the internet gateway.
Variation 2. A company has a VPC with public and private subnets. An EC2 instance in a private subnet needs to download software patches from the internet. Which component should be used to provide internet access to the instance?
medium
✓ A.NAT Gateway in a public subnet
B.AWS Site-to-Site VPN
C.Internet Gateway attached to the VPC
D.VPC Endpoint for Amazon S3
Why A: Option B is correct because a NAT Gateway allows instances in private subnets to initiate outbound traffic to the internet while preventing inbound traffic. Option A is incorrect because an Internet Gateway is used for public subnets, not private. Option C is incorrect because a VPC Endpoint is for accessing AWS services privately. Option D is incorrect because a VPN connection is for on-premises connectivity, not internet access.
Variation 3. A company has a VPC with a public subnet and a private subnet. The private subnet contains an EC2 instance that must access the internet for software updates. Which TWO actions are required to enable this? (Choose TWO.)
medium
A.Add an Internet Gateway to the private subnet's route table.
✓ B.Deploy a NAT Gateway in a public subnet.
C.Assign a public IP address to the EC2 instance.
D.Attach an Internet Gateway to the NAT Gateway.
✓ E.Add a route in the private subnet's route table pointing to the NAT Gateway for 0.0.0.0/0.
Why B: Option A is correct because a NAT Gateway must be in a public subnet. Option C is correct because the private subnet's route table must have a default route to the NAT Gateway. Option B is wrong because the NAT Gateway does not need an Internet Gateway; the public subnet does. Option D is wrong because the EC2 instance does not need a public IP. Option E is wrong because an Internet Gateway is for public subnets, not private.
Variation 4. Which THREE configurations are required to enable an EC2 instance in a private subnet to access the internet for software updates while preventing inbound internet traffic?
hard
✓ A.Attach an Internet Gateway to the VPC.
B.Assign an Elastic IP address to the EC2 instance.
✓ C.Add a route to the private subnet's route table with destination 0.0.0.0/0 pointing to the NAT Gateway.
D.Deploy a bastion host in the private subnet.
✓ E.Place a NAT Gateway in a public subnet.
Why A: A NAT gateway in a public subnet provides outbound internet access. The private subnet route table must have a default route to the NAT gateway. The NAT gateway itself needs an internet gateway in the public subnet route table. Option D is incorrect because an elastic IP is assigned to the NAT gateway, not the instance. Option E is incorrect because a bastion host is for administrative access, not for outbound internet.
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