The answer is the route table of the private subnet does not have a default route pointing to the NAT gateway. For EC2 instances in a private subnet to reach the internet, their route table must include a 0.0.0.0/0 destination that targets the NAT gateway’s elastic network interface; without this entry, traffic from the private subnet has no path to the internet, even though the NAT gateway itself sits in a public subnet with an Internet Gateway. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of VPC routing fundamentals and the distinction between where the NAT gateway lives versus where its route is applied—a common trap is assuming the private subnet inherits routes from the public subnet. Remember the mnemonic: “Private path points to NAT; public path points to IGW.”
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. 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 is using AWS CloudFormation to deploy a multi-tier application. The template includes an Amazon VPC with public and private subnets, NAT gateways, and route tables. After deployment, the EC2 instances in the private subnet cannot access the internet. The NAT gateway is in a public subnet with an Internet Gateway attached. 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 of the private subnet does not have a default route pointing to the NAT Gateway
Option D is correct because the route table associated with the private subnet must have a default route (0.0.0.0/0) pointing to the NAT Gateway. Option A is wrong because security groups can be checked separately. Option B is wrong because NACLs are stateless and would block all traffic if misconfigured. Option C is wrong because Internet Gateway is not needed in the private subnet route; only the NAT Gateway route is needed.
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 route table of the private subnet does not have a default route pointing to the NAT Gateway
Why this is correct
Without a route to the NATGateway, traffic to the internet fails.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The Internet Gateway must be attached, but it is mentioned it is attached.
✗
The security group of the EC2 instances blocks outbound traffic to the internet
Why it's wrong here
Security groups are stateful and allow outbound traffic by default.
✗
The network ACL of the private subnet blocks outbound traffic
Why it's wrong here
NACLs are stateless and would need explicit rules, but the default NACL allows all traffic.
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.
Network Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The route table of the private subnet does not have a default route pointing to the NAT Gateway — Option D is correct because the route table associated with the private subnet must have a default route (0.0.0.0/0) pointing to the NAT Gateway. Option A is wrong because security groups can be checked separately. Option B is wrong because NACLs are stateless and would block all traffic if misconfigured. Option C is wrong because Internet Gateway is not needed in the private subnet route; only the NAT Gateway route is needed.
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.
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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