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Question 601 of 1,705

Network Implementation →mediumMultiple ChoiceObjective-mapped

NAT Gateway Troubleshooting — Common Connectivity Issues

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 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?

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.

A
The security group of the EC2 instances in AZ2 is blocking outbound traffic.
Why wrong: The issue is AZ-specific, not instance-specific.
B
The NAT Gateway in AZ2 does not have an Elastic IP address assigned.
A NAT Gateway requires an Elastic IP; without it, it cannot route traffic to the internet.
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.
Why wrong: Cross-AZ routing works, but the description says the route points to the NAT in the same AZ.
D
The route table for the private subnet in AZ2 is missing a route to the NAT Gateway.
Why wrong: According to the scenario, routes are added.

Full breakdown with real-world context →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The NAT Gateway in AZ2 does not have an Elastic IP address assigned.

The NAT Gateway in AZ2 does not have an Elastic IP address assigned. A NAT Gateway requires an Elastic IP to enable outbound internet traffic. Without it, the NAT Gateway cannot route traffic to the internet, causing the EC2 instances in AZ2 to fail to download updates. Option A is incorrect because security group rules are account-level and would affect both AZs equally. Option C is incorrect because the route table in AZ2 is configured to point to the NAT Gateway in the same AZ, not cross-AZ. Option D is incorrect because the route table for the private subnet in AZ2 does have a route to the NAT Gateway, but the NAT Gateway itself is missing the Elastic IP.

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 of the EC2 instances in AZ2 is blocking outbound traffic.
Why it's wrong here
The issue is AZ-specific, not instance-specific.
✓
The NAT Gateway in AZ2 does not have an Elastic IP address assigned.
Why this is correct
A NAT Gateway requires an Elastic IP; without it, it cannot route traffic to the internet.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
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.
Why it's wrong here
Cross-AZ routing works, but the description says the route points to the NAT in the same AZ.
✗
The route table for the private subnet in AZ2 is missing a route to the NAT Gateway.
Why it's wrong here
According to the scenario, routes are added.

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.

Trap categories for this question

Scenario analysis trap
According to the scenario, routes are added.

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.

Visual reference

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.

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FAQ

Questions learners often ask

What does this ANS-C01 question test?

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 NAT Gateway in AZ2 does not have an Elastic IP address assigned. — The NAT Gateway in AZ2 does not have an Elastic IP address assigned. A NAT Gateway requires an Elastic IP to enable outbound internet traffic. Without it, the NAT Gateway cannot route traffic to the internet, causing the EC2 instances in AZ2 to fail to download updates. Option A is incorrect because security group rules are account-level and would affect both AZs equally. Option C is incorrect because the route table in AZ2 is configured to point to the NAT Gateway in the same AZ, not cross-AZ. Option D is incorrect because the route table for the private subnet in AZ2 does have a route to the NAT Gateway, but the NAT Gateway itself is missing the Elastic IP.

What should I do if I get this ANS-C01 question wrong?

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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Same concept, more angles

2 more ways this is tested on ANS-C01

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 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?

hard

A.The NAT Gateway is deployed in a private subnet
B.The route table for the public subnet in us-east-1b does not have a default route to the Internet Gateway
C.The NAT Gateway has reached the maximum number of concurrent connections
✓ D.The NAT Gateway is in a different Availability Zone than the private subnet instances, causing cross-AZ data transfer charges

Why D: The NAT Gateway is deployed in us-east-1a, but the private subnet in us-east-1b is in a different Availability Zone. While NAT Gateways can forward traffic across AZs, doing so incurs cross-AZ data transfer charges. However, this does not block internet access. The other options are incorrect: A states the NAT Gateway is in a private subnet, which is false; B is irrelevant because the public subnet in us-east-1b does not affect the NAT Gateway's ability to forward traffic; C describes connection limits, which are not mentioned as an issue. Therefore, option D is the most likely cause, as it is the only option that correctly identifies the cross-AZ configuration as a contributor to the reported problem, even though it does not directly prevent internet access.

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 A is correct because a NAT gateway must be placed in a public subnet with a route to an internet gateway (IGW) to translate private IP addresses for outbound traffic. If the NAT gateway is in a private subnet, it cannot reach the IGW, so the private web servers' traffic destined for the internet (via the 0.0.0.0/0 route pointing to the NAT gateway) will fail, as the NAT gateway itself has no path to the internet.

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