The answer is that the firewall rule allow-ssh is missing a source IP range or has a source IP range that does not include the user's IP 203.0.113.5. This is correct because Google Cloud VPC firewall rules are stateful and deny all ingress traffic by default unless an explicit allow rule matches the source, destination, and protocol; without a properly configured source IP range, the SSH traffic from the workstation is silently dropped at the VPC firewall level, even though the rule name exists. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of implicit deny and the critical role of source IP ranges in ingress rules—a common trap is assuming a rule named "allow-ssh" works without verifying its source filter, especially when troubleshooting external connectivity. Remember the memory tip: "Name doesn't filter, source does"—always check the source IP range before blaming the VM or route.
PCNE Implementing network security Practice Question
This PCNE practice question tests your understanding of implementing network security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Network Topology
Refer to the exhibit. A user cannot SSH into test-vm from their workstation (public IP 203.0.113.5) using the VM's external IP 34.67.89.10. The firewall rule allow-ssh exists. 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.
The firewall rule allow-ssh is missing a source IP range or has a source IP range that does not include the user's IP
If the rule does not specify sourceRanges, it defaults to 0.0.0.0/0, but if it was created with an incorrect source range, traffic from 203.0.113.5 would be blocked. The exhibit does not show sourceRanges, but a common misconfiguration is to set sourceRanges to an internal range.
B
The firewall rule allow-ssh is for the wrong network
Why wrong: The rule is for my-vpc, which matches the VM's VPC.
C
The firewall rule allow-ssh is disabled
Why wrong: The output shows DISABLED: False, meaning the rule is enabled.
D
The VM's external IP (34.67.89.10) is blocked by Cloud NAT
Why wrong: Cloud NAT is for outbound traffic, not inbound.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The firewall rule allow-ssh is missing a source IP range or has a source IP range that does not include the user's IP
The most likely cause is that the firewall rule 'allow-ssh' is missing a source IP range or has a source IP range that does not include the user's public IP (203.0.113.5). In Google Cloud, firewall rules are stateful and by default deny all ingress traffic unless explicitly allowed; without a source IP range (or with an incorrect one), the SSH traffic from the user's workstation is dropped at the VPC firewall level, preventing access to the VM's external IP (34.67.89.10).
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 firewall rule allow-ssh is missing a source IP range or has a source IP range that does not include the user's IP
Why this is correct
If the rule does not specify sourceRanges, it defaults to 0.0.0.0/0, but if it was created with an incorrect source range, traffic from 203.0.113.5 would be blocked. The exhibit does not show sourceRanges, but a common misconfiguration is to set sourceRanges to an internal range.
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 firewall rule allow-ssh is for the wrong network
Why it's wrong here
The rule is for my-vpc, which matches the VM's VPC.
✗
The firewall rule allow-ssh is disabled
Why it's wrong here
The output shows DISABLED: False, meaning the rule is enabled.
✗
The VM's external IP (34.67.89.10) is blocked by Cloud NAT
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that a firewall rule's existence alone is sufficient, but the trap here is that the source IP range must be explicitly defined or set to 0.0.0.0/0 for external access; candidates may overlook the source filter configuration and assume the rule name implies it works for all sources.
Trap categories for this question
Command / output trap
The output shows DISABLED: False, meaning the rule is enabled.
Detailed technical explanation
How to think about this question
Google Cloud VPC firewall rules are stateful and evaluated in order of priority (lower number = higher priority), with an implicit deny-all at the end. The source IP range field in an ingress rule is critical: if left empty (0.0.0.0/0), it allows all sources, but if omitted or set to a specific range that excludes the user's IP, the traffic is denied. This is a common misconfiguration when creating rules via the console or gcloud, where the source filter defaults to 'All instances' (which uses tags) rather than 'IP ranges'.
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 firewall rule allow-ssh is missing a source IP range or has a source IP range that does not include the user's IP — The most likely cause is that the firewall rule 'allow-ssh' is missing a source IP range or has a source IP range that does not include the user's public IP (203.0.113.5). In Google Cloud, firewall rules are stateful and by default deny all ingress traffic unless explicitly allowed; without a source IP range (or with an incorrect one), the SSH traffic from the user's workstation is dropped at the VPC firewall level, preventing access to the VM's external IP (34.67.89.10).
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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