The answer is the missing default route in the private subnet’s route table. For EC2 instances in a private subnet to reach the internet, their route table must include a default route (0.0.0.0/0) pointing to the NAT Gateway. Without this route, traffic destined for the internet has no path to the NAT Gateway, causing connectivity failure—even though the NAT Gateway itself is correctly placed in a public subnet. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of VPC routing fundamentals and the distinction between public and private subnet configurations. A common trap is assuming the NAT Gateway’s presence alone is sufficient, or confusing this with NACL rules, which are stateless and would affect all subnets equally. Remember the memory tip: “Private subnet, public route—point 0.0.0.0/0 to the NAT to get out.”
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 across three Availability Zones. The public subnets host NAT Gateways, and the private subnets host EC2 instances that need to access the internet. The SysOps administrator notices that EC2 instances in one private subnet cannot reach the internet, while others can. 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 route table for the private subnet does not have a default route to the NAT Gateway.
Option C is correct. The route table for the private subnet must have a default route (0.0.0.0/0) pointing to the NAT Gateway. If missing, internet access fails. Option A is wrong because NAT Gateways are in public subnets. Option B is wrong because NACLs are stateless but would affect all instances. Option D is wrong because the secondary private IP is irrelevant.
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 EC2 instances have a secondary private IP address that is not registered.
Why it's wrong here
Secondary IP does not affect internet routing.
✗
The NAT Gateway is not in a public subnet.
Why it's wrong here
NAT Gateways must be in a public subnet, but that would affect all subnets equally.
✗
The network ACL for the private subnet blocks outbound traffic.
Why it's wrong here
NACLs are stateless and would affect all instances in that subnet, but the question says only one subnet is affected.
✓
The route table for the private subnet does not have a default route to the NAT Gateway.
Why this is correct
Missing route to NATGateway prevents internet access.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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 C is correct. The route table for the private subnet must have a default route (0.0.0.0/0) pointing to the NAT Gateway. If missing, internet access fails. Option A is wrong because NAT Gateways are in public subnets. Option B is wrong because NACLs are stateless but would affect all instances. Option D is wrong because the secondary private IP is irrelevant.
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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