The answer is that the route table for the public subnet in us-east-1b does not have a default route to the Internet Gateway. This is the most likely cause because while the private subnet instances in us-east-1b can route traffic to the NAT Gateway in us-east-1a across Availability Zones, the NAT Gateway itself must then forward that traffic to the Internet Gateway—but it can only do so if the public subnet where it resides has a proper egress route. When troubleshooting NAT gateway connectivity issues cross availability zone, remember that the NAT Gateway’s own subnet must have a 0.0.0.0/0 route to the Internet Gateway, regardless of which AZ the private instances are in. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding that NAT Gateways are AZ-resilient but not AZ-agnostic; a common trap is assuming the private subnet route alone is sufficient. Memory tip: “The NAT needs a road to the IGW—check the public subnet route table, not just the private one.”
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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 CIDR 10.0.0.0/16. They have two Availability Zones (us-east-1a and us-east-1b). In each AZ, there is a public subnet (10.0.1.0/24 and 10.0.2.0/24) and a private subnet (10.0.3.0/24 and 10.0.4.0/24). A NAT Gateway is deployed in the public subnet of us-east-1a. The private route tables for both private subnets have a default route pointing to the NAT Gateway. An application team has deployed EC2 instances in the private subnets. They report that instances in us-east-1b cannot access the internet, while instances in us-east-1a can. The NAT Gateway is healthy and has an Elastic IP attached. The route tables for the public subnets have a default route to the Internet Gateway. What is the most likely cause of the issue?
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 public subnet in us-east-1b does not have a default route to the Internet Gateway
Option B is correct because the NATGateway is only in us-east-1a, so traffic from us-east-1b private subnets must traverse an Availability Zone boundary. The route table for the private subnet in us-east-1b points to the NAT Gateway, but the NAT Gateway is in a different AZ. However, that is allowed. The issue is likely the public subnet route table for us-east-1b does not have a route to the Internet Gateway, so the NAT Gateway cannot send traffic out. Option A is wrong because cross-AZ traffic is allowed. Option C is wrong because there is no such limit. Option D is wrong because NAT Gateways are not assigned to private subnets.
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 NAT Gateway has reached the maximum number of concurrent connections
Why it's wrong here
No evidence of connection limits being reached.
✗
The NAT Gateway is in a different Availability Zone than the private subnet instances, causing cross-AZ data transfer charges
Why it's wrong here
Cross-AZ traffic is allowed and charges are not the issue.
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 ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The route table for the public subnet in us-east-1b does not have a default route to the Internet Gateway — Option B is correct because the NAT Gateway is only in us-east-1a, so traffic from us-east-1b private subnets must traverse an Availability Zone boundary. The route table for the private subnet in us-east-1b points to the NAT Gateway, but the NAT Gateway is in a different AZ. However, that is allowed. The issue is likely the public subnet route table for us-east-1b does not have a route to the Internet Gateway, so the NAT Gateway cannot send traffic out. Option A is wrong because cross-AZ traffic is allowed. Option C is wrong because there is no such limit. Option D is wrong because NAT Gateways are not assigned to private subnets.
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.
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 CIDR of 10.0.0.0/16. They have two Availability Zones, each with a public subnet (10.0.1.0/24 and 10.0.2.0/24) and a private subnet (10.0.3.0/24 and 10.0.4.0/24). They have an internet-facing ALB in the public subnets and EC2 instances in the private subnets. The EC2 instances need to download updates from the internet. They deploy a NAT Gateway in each public subnet and add routes in the private subnet route tables pointing to the respective NAT Gateway in the same AZ. However, the EC2 instances in AZ2 cannot access the internet, while those in AZ1 can. What is the most likely cause?
medium
A.The security group of the EC2 instances in AZ2 is blocking outbound traffic.
✓ B.The NAT Gateway in AZ2 does not have an Elastic IP address assigned.
C.The private subnet in AZ2 is routing traffic to the NAT Gateway in AZ1, which is in a different Availability Zone and incurs cross-AZ charges but should still work.
D.The route table for the private subnet in AZ2 is missing a route to the NAT Gateway.
Why B: The NAT Gateway in AZ2 may be in a different public subnet than the one the private subnet route points to, or the NAT Gateway may not have an associated Elastic IP. Option A is wrong because cross-AZ routing is possible but not the issue. Option B is wrong because the route is to the specific NAT Gateway in the same AZ. Option D is wrong because the issue is specific to one AZ, not a global issue.
Variation 2. A company is deploying a VPC with public and private subnets in two Availability Zones. The public subnets contain NAT gateways for outbound internet access from the private subnets. The private subnets host web servers that need to make API calls to an external service over the internet. After implementation, the web servers cannot reach the internet. Which configuration is the most likely cause?
medium
✓ A.The NAT gateway is placed in a private subnet and does not have a route to the internet gateway.
B.The route table for the private subnets is not associated with the VPC's main route table.
C.The private subnets have a default route (0.0.0.0/0) pointing to an internet gateway instead of the NAT gateway.
D.The public subnets have a default route pointing to the NAT gateway instead of the internet gateway.
Why A: Option B is correct because each NAT gateway must be in a public subnet with a route to an internet gateway. If the NAT gateway is in a private subnet, it cannot route traffic to the internet. Option A is incorrect because private subnets do not need default routes to the internet gateway. Option C is incorrect because route propagation is not required. Option D is incorrect because NAT gateway is not a required check for public subnets.
Last reviewed: Jun 20, 2026
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This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.
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