The answer is to verify that the route table is correctly associated with the private subnet. Even when a route table contains a default route (0.0.0.0/0) pointing to a NAT Gateway, that route only applies if the route table is actually attached to the subnet hosting the web application. A common misconfiguration is creating the correct route table but failing to associate it with the target private subnet, leaving the subnet using the VPC’s default route table, which typically lacks the NAT Gateway path. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of route table association as a distinct step from route creation—a frequent trap where engineers assume a route exists in the VPC and forget to link it to the specific subnet. The exam emphasizes that private subnet internet access via NAT gateway route table association requires both the correct route entry and the explicit subnet attachment. Memory tip: “A route without a subnet is just a map without a destination—associate it to make the connection.”
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 has a VPC with a public subnet and a private subnet. The private subnet hosts a web application that needs to access an external API over the internet. The private subnet uses a NAT Gateway in the public subnet for outbound internet access. The web application is failing to reach the external API. The engineer has verified the following: the NAT Gateway has an Elastic IP attached, the route table for the private subnet has a default route (0.0.0.0/0) pointing to the NAT Gateway, the security group for the web application allows outbound HTTPS (TCP 443) to 0.0.0.0/0, the network ACL for the private subnet allows inbound and outbound TCP ephemeral ports (1024-65535) from and to 0.0.0.0/0, and the IAM role attached to the EC2 instance allows outbound HTTPS. The engineer also confirmed that the NAT Gateway is in the public subnet which has a route to an Internet Gateway. Despite all these checks, the web application still cannot reach the external API. What should the engineer do next?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Verify that the route table is correctly associated with the private subnet.
Option D is correct because even though the route table has a default route to the NAT Gateway, the subnet association might be incorrect. The route table associated with the private subnet might not be the one being used. Checking the subnet association ensures that the correct route table is applied. Option A is incorrect because the NAT Gateway is already in the public subnet. Option B is incorrect because the default route is present. Option C is incorrect because the security group already allows outbound HTTPS.
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.
✗
Add an inbound rule to the security group for the external API's IP address.
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 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: Verify that the route table is correctly associated with the private subnet. — Option D is correct because even though the route table has a default route to the NAT Gateway, the subnet association might be incorrect. The route table associated with the private subnet might not be the one being used. Checking the subnet association ensures that the correct route table is applied. Option A is incorrect because the NAT Gateway is already in the public subnet. Option B is incorrect because the default route is present. Option C is incorrect because the security group already allows outbound HTTPS.
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.
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. The private subnet instances need to make outbound internet requests. A NAT Gateway is deployed in the public subnet. The network engineer notices that instances in the private subnet cannot reach the internet, but the NAT Gateway's Elastic IP is reachable from the internet. Which of the following is the most likely cause?
hard
A.The network ACL on the private subnet blocks outbound traffic.
✓ B.The route table for the private subnet does not have a default route (0.0.0.0/0) pointing to the NAT Gateway.
C.The NAT Gateway is not associated with an Elastic IP.
D.The security group attached to the NAT Gateway blocks outbound traffic.
Why B: Option C is correct because the private subnet route table must have a default route (0.0.0.0/0) pointing to the NAT Gateway. Option A is wrong because the NAT Gateway is in a public subnet with a route to IGW. Option B is wrong because security groups are not applicable to NAT Gateway. Option D is wrong because NACL on private subnet would affect inbound/outbound traffic, but the issue is routing.
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Question Discussion
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