- A
Firewall rules allow ingress from IAP's IP ranges only for some VMs.
Correct. If firewall rules allowing ingress from IAP's IP ranges are not applied to all VMs (despite same network tags), those VMs become inaccessible via IAP tunnel.
- B
The VMs are in different zones with different DNS resolution.
Why wrong: Incorrect. DNS resolution is consistent within a project zone, and zones do not affect IAP connectivity.
- C
The VMs that can be accessed have external IP addresses; the others only have internal IP addresses.
Why wrong: Incorrect. IAP TCP forwarding does not require the target VM to have an external IP address; it connects to the internal IP via the IAP proxy.
- D
The VMs do not have OS Login enabled.
Why wrong: Incorrect. OS Login is optional; IAP TCP forwarding works with standard SSH keys as well.
PCNE IAP TCP Forwarding Practice Question
This PCNE practice question tests your understanding of implementing network security. 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. A key principle to apply: iAP TCP Forwarding. 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 Identity-Aware Proxy (IAP) to secure access to Compute Engine VMs. Users report that they can SSH into some VMs but not others, even though they have the IAP-secured Tunnel User role. Both VMs are in the same project and have the same network tags. What is the most likely reason?
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 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
Firewall rules allow ingress from IAP's IP ranges only for some VMs.
Option A is correct because IAP TCP forwarding relies on firewall rules that allow ingress from the IAP proxy's IP ranges (35.235.240.0/20) to the VM. If these rules are applied inconsistently across VMs (e.g., missing on some subnets or instances), users will be unable to SSH into those VMs. The fact that both VMs share the same network tags does not guarantee identical firewall configurations if rules are applied based on other criteria like target tags or service accounts. Options B and D are incorrect as DNS resolution and OS Login are not prerequisites for IAP TCP forwarding. Option C is wrong because IAP TCP forwarding works with internal IPs; external IPs are not required.
Key principle: IAP TCP Forwarding
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Firewall rules allow ingress from IAP's IP ranges only for some VMs.
Why this is correct
Correct. If firewall rules allowing ingress from IAP's IP ranges are not applied to all VMs (despite same network tags), those VMs become inaccessible via IAP tunnel.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
IAP TCP Forwarding
- ✗
The VMs are in different zones with different DNS resolution.
Why it's wrong here
Incorrect. DNS resolution is consistent within a project zone, and zones do not affect IAP connectivity.
- ✗
The VMs that can be accessed have external IP addresses; the others only have internal IP addresses.
- ✗
The VMs do not have OS Login enabled.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is assuming that identical IAM roles and network tags guarantee access, while overlooking that IAP TCP forwarding also requires explicit firewall rules allowing ingress from the IAP proxy IP ranges. Candidates often forget to verify firewall rules.
Detailed technical explanation
How to think about this question
IAP for TCP forwarding works by creating a tunnel from the user's device through the IAP service to the VM's external IP address on port 22 (or 3389 for RDP). The VM must have an external IP (even if ephemeral) or be behind a Cloud NAT with appropriate routes so that the IAP service can reach it; internal-only VMs cannot be targeted because IAP does not have a path to private IPs. This is a common pitfall when designing secure bastionless access, as teams often assume IAP works with internal IPs alone.
KKey Concepts to Remember
- IAP TCP Forwarding
- Firewall Rules for IAP
- Network Tags and Firewall Rules
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
IAP TCP Forwarding
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.
Visual reference
What to study next
Got this wrong? Here's your next step.
Review iAP TCP Forwarding, then practise related PCNE questions on the same topic to reinforce the concept.
- →
Implementing network security — study guide chapter
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FAQ
Questions learners often ask
What does this PCNE question test?
Implementing network security — This question tests Implementing network security — IAP TCP Forwarding.
What is the correct answer to this question?
The correct answer is: Firewall rules allow ingress from IAP's IP ranges only for some VMs. — Option A is correct because IAP TCP forwarding relies on firewall rules that allow ingress from the IAP proxy's IP ranges (35.235.240.0/20) to the VM. If these rules are applied inconsistently across VMs (e.g., missing on some subnets or instances), users will be unable to SSH into those VMs. The fact that both VMs share the same network tags does not guarantee identical firewall configurations if rules are applied based on other criteria like target tags or service accounts. Options B and D are incorrect as DNS resolution and OS Login are not prerequisites for IAP TCP forwarding. Option C is wrong because IAP TCP forwarding works with internal IPs; external IPs are not required.
What should I do if I get this PCNE question wrong?
Review iAP TCP Forwarding, then practise related PCNE questions on the same topic to reinforce the concept.
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?
IAP TCP Forwarding
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Last reviewed: Jun 25, 2026
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.
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