The correct answer is that the VM has a custom route which does not use the default route through Cloud NAT. Cloud NAT functions by relying on a default route (0.0.0.0/0) that points to the Cloud Router, ensuring all outbound traffic from private instances is source-NATed to the NAT gateway’s IP. When a custom route with a more specific destination or a different next hop is applied to a VM, it overrides that default route, causing traffic to bypass Cloud NAT entirely and exit with the VM’s private IP. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of route priority and how Cloud NAT depends on the default route—a common trap is assuming NAT applies automatically regardless of routing. Remember the key rule: Cloud NAT is a routing-based service, not a per-VM setting; if the packet doesn’t hit the 0.0.0.0/0 route, it won’t be NATed. Memory tip: “No default route, no NAT—custom paths mean private IPs.”
PCNE Implementing network security Practice Question
This PCNE practice question tests your understanding of implementing network security. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 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?
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.
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
Option D is correct because 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.
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 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.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
Cloud NAT performs source network address translation (SNAT) on packets that match the default route (0.0.0.0/0) via the Cloud Router's next hop. If a VM has a custom static route with a destination prefix that overlaps with the external IP (e.g., a specific /32 route), the packet will be forwarded according to that route instead of the default route, bypassing Cloud NAT. This is a common misconfiguration when using policy-based routing or when VPC peering introduces conflicting routes. The VM's source IP remains its private IP, causing external firewalls to reject the traffic as the source IP does not match the expected NAT IP.
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.
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.
Implementing network security — This question tests Implementing network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The VM has a custom route that does not use the default route through Cloud NAT — Option D is correct because 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.
What should I do if I get this PCNE 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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Question Discussion
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