ANS-C01 Network Management and Operations Practice Question
A company has a VPC with a CIDR of 10.0.0.0/16. They have two subnets: 10.0.1.0/24 (public) and 10.0.2.0/24 (private). An EC2 instance in the private subnet needs to download patches from the internet. The team creates a NAT gateway in the public subnet and updates the private subnet route table to route 0.0.0.0/0 to the NAT gateway. The EC2 instance still cannot reach the internet. The team confirms the NAT gateway has an elastic IP and the public subnet has an internet gateway. What is the MOST likely cause?
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 network ACL for the private subnet denies outbound traffic to the internet.
The most likely cause is that the network ACL (NACL) for the private subnet denies outbound traffic to the internet. Even though the route table correctly points 0.0.0.0/0 to the NAT gateway, the NACL is a stateless firewall that can block outbound traffic if it doesn't allow the necessary ephemeral ports. It must also allow inbound return traffic. Option A is incorrect because the private subnet route table already has a route to the NAT gateway. Option C is incorrect because the public subnet already has a route to the internet gateway. Option D is incorrect because the NAT gateway is a managed service with source/destination check automatically disabled.
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 private subnet route table does not have a route for the NAT gateway's IP.
Why it's wrong here
Route is for 0.0.0.0/0 to NAT gateway, not its IP.
- ✓
The network ACL for the private subnet denies outbound traffic to the internet.
Why this is correct
Network ACL must allow outbound ephemeral ports and inbound return traffic.
- ✗
The public subnet does not have a route to the internet gateway.
Why it's wrong here
Public subnet must have that route; but issue is from private subnet.
- ✗
The NAT gateway does not have source/destination check disabled.
Why it's wrong here
NAT gateway is managed; source/destination check is disabled by default.
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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.