PCNE Configuring network services Practice Question
This PCNE practice question tests your understanding of configuring network services. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 uses Cloud NAT to allow private VMs to access the internet. They notice that some VMs are unable to reach a specific set of external IP addresses, but other VMs can. The firewall rules are correctly configured. 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 Cloud NAT gateway uses a static IP address that is not allowed by the external service's firewall.
Cloud NAT uses a source IP address (either a static IP you assign or an ephemeral IP from the NAT gateway's IP range) when translating outbound traffic from private VMs. If the external service's firewall only allows traffic from specific IP addresses, and the Cloud NAT gateway is using a static IP that is not on that allowlist, the affected VMs' traffic will be blocked. Other VMs might reach the service if they use a different NAT gateway or if the service's firewall permits their translated 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 default route (0.0.0.0/0) is missing for the VPC.
Why it's wrong here
Missing default route would affect all VMs, not just some.
✗
The VPC firewall rules have a deny rule for the source IP range of the affected VMs.
Why it's wrong here
Firewall rules apply to VM instances, but the issue is with external reachability, not internal.
✓
The Cloud NAT gateway uses a static IP address that is not allowed by the external service's firewall.
Why this is correct
If the external service restricts access by IP, only VMs using that NAT IP can connect.
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.
✗
The VMs have a tag that overrides the Cloud NAT routing.
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that firewall rules within the VPC are the only cause of connectivity failures, when in reality external service firewalls or IP allowlists can block traffic after NAT translation.
Detailed technical explanation
How to think about this question
Cloud NAT performs source network address translation (SNAT) using either a static IP address (reserved external IP) or an ephemeral IP from the NAT gateway's IP range. When an external service implements IP-based access control (e.g., an allowlist), the source IP seen by the service is the NAT gateway's IP, not the private VM's IP. If the NAT gateway's IP is not on the allowlist, traffic is dropped at the service's firewall, even though the VPC firewall rules are permissive. This is a common issue when migrating to Cloud NAT from individual external IPs on VMs.
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.
Configuring network services — This question tests Configuring network services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Cloud NAT gateway uses a static IP address that is not allowed by the external service's firewall. — Cloud NAT uses a source IP address (either a static IP you assign or an ephemeral IP from the NAT gateway's IP range) when translating outbound traffic from private VMs. If the external service's firewall only allows traffic from specific IP addresses, and the Cloud NAT gateway is using a static IP that is not on that allowlist, the affected VMs' traffic will be blocked. Other VMs might reach the service if they use a different NAT gateway or if the service's firewall permits their translated 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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