ANS-C01 Network Management and Operations Practice Question
A company has a VPC with public and private subnets. The private subnets need to access the internet for software updates. The company has set up a NAT gateway in the public subnet. However, instances in the private subnet cannot reach the internet. 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 private subnet route table does not have a route to the NAT gateway
Instances in a private subnet require a route to the NAT gateway to reach the internet. The private subnet's route table must have a default route (0.0.0.0/0) pointing to the NAT gateway as its target. Without this route, traffic from private instances sent to the internet will not be directed to the NAT gateway, and will instead remain in the VPC or be dropped. Option A is incorrect because the NAT gateway itself resides in a public subnet and uses the internet gateway via that subnet's route table; it does not require its own route to the IGW. Option B is incorrect because security groups by default allow all outbound traffic, so this is unlikely to be the issue. Option C is incorrect because not having an Elastic IP would prevent the NAT gateway from having a public IP, but the question states a NAT gateway is set up, implying it has an Elastic IP; the more fundamental issue is the missing route.
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 NAT gateway does not have a route to the internet gateway
Why it's wrong here
NAT gateway in public subnet automatically uses the internet gateway; no explicit route needed.
- ✗
The security group for the instances blocks outbound traffic
Why it's wrong here
Security groups allow outbound traffic by default unless explicitly denied.
- ✗
The NAT gateway is not associated with an Elastic IP
Why it's wrong here
NAT gateway requires an Elastic IP, but the question implies it is set up, so this is likely not the issue.
- ✓
The private subnet route table does not have a route to the NAT gateway
Why this is correct
Without a default route to the NAT gateway, traffic from private subnet cannot reach the internet.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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