PCNE Implementing network security Practice Question
A company is using Cloud NAT for internet access from private subnets. Security team notices that traffic from a specific VM is being blocked by external firewalls because the source IP is not the Cloud NAT IP. What is the most likely cause?
⚠ Common exam trap
Google Cloud often tests the misconception that Cloud NAT is zone-dependent or that firewall rules are the cause, when in reality the issue is almost always a routing override that prevents traffic from reaching the NAT gateway.
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 VM has a custom route that does not use the default route through Cloud NAT
Cloud NAT relies on the default route (0.0.0.0/0) pointing to the Cloud Router to direct traffic through the NAT gateway. If a VM has a custom route that overrides the default route (e.g., a more specific route to an external IP or a route to a different next hop), the VM's outbound traffic will bypass Cloud NAT entirely, resulting in the source IP being the VM's private IP instead of the Cloud NAT IP. This causes external firewalls to block the traffic as the source IP is not the expected NAT IP.
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 VM is in a different zone than the Cloud NAT gateway
Why it's wrong here
Cloud NAT is regional and works for all zones within that region.
- ✗
The VPC firewall rules are blocking outbound traffic from the VM to the Cloud NAT IP
Why it's wrong here
Firewall rules do not affect NAT translation; they control traffic flow.
- ✗
Cloud Router is misconfigured and not advertising the Cloud NAT IP
Why it's wrong here
Cloud Router is used for BGP, not for NAT translation.
- ✓
The VM has a custom route that does not use the default route through Cloud NAT
Why this is correct
Traffic must match the default route to be source NATed by Cloud NAT.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PCNE practice question is part of Courseiva's free Google Cloud 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 PCNE exam.