The answer is a missing default route to the NAT Gateway in the private subnet’s route table. Even though the EC2 instance can ping the NAT Gateway—because that traffic stays within the VPC—any outbound internet-bound packet must match a 0.0.0.0/0 route pointing to the NAT Gateway to be forwarded correctly. Without that route, the instance has no path to the internet, which is why the software download fails despite the ping succeeding. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding that routing, not security groups or NACLs, is the root cause when private subnet internet access fails. A common trap is assuming a successful ping to the NAT Gateway proves internet connectivity, but ping only confirms local reachability. Remember: for private subnet internet access, the NAT Gateway route must be the default route—if it’s missing, traffic goes nowhere.
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 a CIDR block of 10.0.0.0/16. The VPC has two public subnets (10.0.1.0/24 and 10.0.2.0/24) and two private subnets (10.0.3.0/24 and 10.0.4.0/24). The VPC has an Internet Gateway (IGW) attached. The public subnets have a route to the IGW, and the private subnets have a route to a NAT Gateway in the public subnet 10.0.1.0/24. The SysOps administrator deploys a new EC2 instance in the private subnet 10.0.4.0/24. The instance needs to download software from the internet, but the download fails. The administrator can successfully ping the NAT Gateway from the instance. What is the MOST likely cause of the failure?
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 route table for the private subnet does not have a default route to the NAT Gateway.
Option D is correct. The route table for the private subnet 10.0.4.0/24 is missing a default route (0.0.0.0/0) to the NAT Gateway. The ping to the NAT Gateway works because it is within the VPC, but internet traffic is not routed. Option A is wrong because the NAT Gateway is in the public subnet and has internet access. Option B is wrong because the security group only controls inbound/outbound for the instance, not routing. Option C is wrong because the NACL for the private subnet should allow outbound traffic, but the issue is routing.
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 security group for the EC2 instance blocks outbound internet traffic.
Why it's wrong here
Security groups default allow all outbound.
✓
The route table for the private subnet does not have a default route to the NAT Gateway.
Why this is correct
Missing route prevents internet-bound traffic from reaching the NATGateway.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The network ACL for the private subnet blocks outbound traffic.
Why it's wrong here
NACL would block all instances, but the issue is specific to one subnet.
✗
The NAT Gateway is not associated with an Elastic IP address.
Why it's wrong here
NAT Gateways require an EIP, but the ping succeeded, so it likely has one.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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 route table for the private subnet does not have a default route to the NAT Gateway. — Option D is correct. The route table for the private subnet 10.0.4.0/24 is missing a default route (0.0.0.0/0) to the NAT Gateway. The ping to the NAT Gateway works because it is within the VPC, but internet traffic is not routed. Option A is wrong because the NAT Gateway is in the public subnet and has internet access. Option B is wrong because the security group only controls inbound/outbound for the instance, not routing. Option C is wrong because the NACL for the private subnet should allow outbound traffic, but the issue is routing.
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.
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