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Question 1,240 of 1,705

Network Management and Operations →hardMultiple ChoiceObjective-mapped

NAT Gateway Private Subnet No Internet: Missing Default Route

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 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?

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 network ACL on the private subnet blocks outbound traffic.
Why wrong: NACL would block all outbound, but the issue is likely routing.
B
The route table for the private subnet does not have a default route (0.0.0.0/0) pointing to the NAT Gateway.
Without this route, outbound traffic cannot reach the NAT Gateway.
C
The NAT Gateway is not associated with an Elastic IP.
Why wrong: Stem says EIP is reachable, so it is associated.
D
The security group attached to the NAT Gateway blocks outbound traffic.
Why wrong: NAT Gateway does not have security groups.

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 route table for the private subnet does not have a default route (0.0.0.0/0) pointing to the NAT Gateway.

The private subnet instances cannot reach the internet because the route table for the private subnet does not have a default route (0.0.0.0/0) pointing to the NAT Gateway. Without this route, traffic destined for the internet has no path. Option A is incorrect because a NACL blocking outbound traffic would cause a different symptom, but the stem indicates the NAT Gateway's EIP is reachable, suggesting routing is the issue. Option C is incorrect because the NAT Gateway is already associated with an Elastic IP. Option D is incorrect because security groups are not applied to NAT Gateways; they are applied to instances.

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 network ACL on the private subnet blocks outbound traffic.
Why it's wrong here
NACL would block all outbound, but the issue is likely routing.
✓
The route table for the private subnet does not have a default route (0.0.0.0/0) pointing to the NAT Gateway.
Why this is correct
Without this route, outbound traffic cannot reach the NAT Gateway.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The NAT Gateway is not associated with an Elastic IP.
Why it's wrong here
Stem says EIP is reachable, so it is associated.
✗
The security group attached to the NAT Gateway blocks outbound traffic.
Why it's wrong here
NAT Gateway does not have security groups.

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.

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 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 for the private subnet does not have a default route (0.0.0.0/0) pointing to the NAT Gateway. — The private subnet instances cannot reach the internet because the route table for the private subnet does not have a default route (0.0.0.0/0) pointing to the NAT Gateway. Without this route, traffic destined for the internet has no path. Option A is incorrect because a NACL blocking outbound traffic would cause a different symptom, but the stem indicates the NAT Gateway's EIP is reachable, suggesting routing is the issue. Option C is incorrect because the NAT Gateway is already associated with an Elastic IP. Option D is incorrect because security groups are not applied to NAT Gateways; they are applied to instances.

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

1 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 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?

hard

A.Add an inbound rule to the security group for the external API's IP address.
B.Move the NAT Gateway to the private subnet.
✓ C.Verify that the route table is correctly associated with the private subnet.
D.Add a specific route for the external API's IP address to the route table.

Why C: Option C is correct because even though the route table contains a default route to the NAT Gateway, it may not be associated with the private subnet. If the route table is not associated, the private subnet uses the main route table, which might not have the default route. The engineer should verify that the correct route table is associated with the private subnet. Option A is incorrect because the security group already allows outbound HTTPS. Option B is incorrect because the NAT Gateway must be in a public subnet. Option D is incorrect because a specific route is unnecessary when a default route already exists.

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