ANS-C01 Network Security, Compliance and Governance Practice Question
A company has a VPC with CIDR 10.0.0.0/16. They have public subnets (10.0.1.0/24, 10.0.2.0/24) and private subnets (10.0.3.0/24, 10.0.4.0/24) in two Availability Zones. An Internet Gateway is attached to the VPC, and a NAT Gateway is in public subnet 10.0.1.0/24. The private subnets route 0.0.0.0/0 to the NAT Gateway. The security team notices that instances in private subnet 10.0.3.0/24 can reach the internet, but instances in private subnet 10.0.4.0/24 cannot. Both private subnets have the same route table configuration. The network ACLs for both private subnets are set to allow all inbound and outbound traffic. What is the most likely cause of the issue?
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 public subnet 10.0.1.0/24 does not have a route to the Internet Gateway.
The NAT Gateway is in the public subnet 10.0.1.0/24. For the NAT Gateway to reach the internet, the route table associated with that public subnet must have a default route (0.0.0.0/0) pointing to the Internet Gateway. Without this route, the NAT Gateway cannot forward traffic from private subnets to the internet. Since both private subnets have the same route table configuration pointing to the NAT Gateway, the issue is isolated to the NAT Gateway's inability to reach the internet due to the missing route in the public subnet's route table. Options B, C, and D are incorrect: security groups and network ACLs are configured to allow traffic, and the private subnet route tables are correctly set to route to the NAT Gateway.
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 table for the public subnet 10.0.1.0/24 does not have a route to the Internet Gateway.
Why this is correct
The NAT Gateway needs a route to the Internet Gateway through its subnet's route table.
- ✗
The security group on the instances in private subnet 10.0.4.0/24 is blocking outbound traffic.
Why it's wrong here
Security groups would affect both subnets similarly if applied.
- ✗
The network ACL for private subnet 10.0.4.0/24 is blocking outbound traffic.
Why it's wrong here
Network ACLs are set to allow all traffic.
- ✗
The route table for private subnet 10.0.4.0/24 does not have a route to the NAT Gateway.
Why it's wrong here
Both private subnets share the same route table, so they have the same route.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.