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Question 148 of 1,748

Infrastructure Security →hardMultiple ChoiceObjective-mapped

Why EC2 Instances Behind NAT Gateway Have No Internet: Public Subnet Needs 0.0.0.0/0 to IGW

This SCS-C02 practice question tests your understanding of infrastructure security. 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 public subnets (10.0.1.0/24 and 10.0.2.0/24) and two private subnets (10.0.3.0/24 and 10.0.4.0/24). They have an Application Load Balancer in the public subnets and EC2 instances in the private subnets. The EC2 instances need to access the internet for updates. The security engineer has set up a NAT gateway in each public subnet. The route table for the private subnets has a default route pointing to the NAT gateway in the same Availability Zone. However, the EC2 instances are unable to reach the internet. 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 route tables for the public subnets do not have a default route (0.0.0.0/0) to the Internet Gateway.
NAT gateways need a route to IGW to reach the internet.
B
The security groups for the EC2 instances do not allow outbound traffic.
Why wrong: Security groups allow all outbound traffic by default.
C
The NAT gateways do not have route propagation enabled.
Why wrong: Route propagation is for VPN connections.
D
The NAT gateways are in the same Availability Zone.
Why wrong: They can be in different AZs; this is not a problem.

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 tables for the public subnets do not have a default route (0.0.0.0/0) to the Internet Gateway.

The most likely cause is that the public subnets' route tables lack a default route (0.0.0.0/0) pointing to the Internet Gateway (IGW). For a NAT gateway to function, it must be placed in a public subnet with a route to the IGW so it can translate private IPs and forward traffic to the internet. Without this route, the NAT gateway cannot send traffic to the IGW, breaking outbound connectivity for EC2 instances in private subnets that rely on it.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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 tables for the public subnets do not have a default route (0.0.0.0/0) to the Internet Gateway.
Why this is correct
NAT gateways need a route to IGW to reach the internet.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The security groups for the EC2 instances do not allow outbound traffic.
Why it's wrong here
Security groups allow all outbound traffic by default.
✗
The NAT gateways do not have route propagation enabled.
Why it's wrong here
Route propagation is for VPN connections.
✗
The NAT gateways are in the same Availability Zone.
Why it's wrong here
They can be in different AZs; this is not a problem.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates assume NAT gateways automatically have internet access if placed in a public subnet, but they forget that the public subnet's route table must explicitly have a default route to the Internet Gateway for the NAT gateway to function.

Detailed technical explanation

How to think about this question

NAT gateways operate at the network layer (Layer 3) and perform source NAT (SNAT), rewriting the source IP of outbound packets to the NAT gateway's Elastic IP. For this to work, the NAT gateway must have a route to the IGW in its subnet's route table; otherwise, packets destined for 0.0.0.0/0 are dropped. Additionally, the private subnet's route table must point to the NAT gateway's ENI, not the IGW, to ensure traffic is routed through the NAT gateway for translation.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.

TExam Day Tips

Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this SCS-C02 question test?

Infrastructure Security — This question tests Infrastructure Security — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The route tables for the public subnets do not have a default route (0.0.0.0/0) to the Internet Gateway. — The most likely cause is that the public subnets' route tables lack a default route (0.0.0.0/0) pointing to the Internet Gateway (IGW). For a NAT gateway to function, it must be placed in a public subnet with a route to the IGW so it can translate private IPs and forward traffic to the internet. Without this route, the NAT gateway cannot send traffic to the IGW, breaking outbound connectivity for EC2 instances in private subnets that rely on it.

What should I do if I get this SCS-C02 question wrong?

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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?

Read the scenario before looking for a memorised answer.

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