The correct design is to configure Cloud NAT and add firewall rules to allow only necessary egress and deny all ingress. Cloud NAT enables outbound internet connectivity for Compute Engine instances without external IPs by translating their private addresses to a single public IP, while the explicit firewall rules ensure that no inbound traffic can reach those instances, even from the NAT gateway’s IP. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of how Cloud NAT decouples outbound access from inbound exposure, and a common trap is assuming default VPC firewall rules are sufficient—they allow some ingress like ICMP, so you must override them with a deny-all-ingress rule to fully block inbound traffic. Remember the key principle: Cloud NAT gives you outbound without inbound, but you still need to lock the door with egress-only firewall rules. Memory tip: “NAT out, deny in” — Cloud NAT handles the out, a deny-all-ingress rule handles the in.
PCNE Practice Question: Designing, planning, and prototyping a GCP network
This PCNE practice question tests your understanding of designing, planning, and prototyping a gcp network. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 has Compute Engine instances without external IPs that need to access the internet for updates. They do not want any inbound traffic. What is the best design?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure Cloud NAT and add firewall rules to allow only necessary egress and deny all ingress.
Option B is correct because Cloud NAT provides outbound internet connectivity for instances without external IPs, and the explicit firewall rules ensure only necessary egress traffic is allowed while denying all ingress, meeting the requirement of no inbound traffic. Default firewall rules allow some ingress (e.g., ICMP), so they must be overridden with a deny-all-ingress rule to fully block inbound traffic.
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.
✗
Configure Cloud NAT and rely on default firewall rules.
Why it's wrong here
Default rules allow inbound from inside VPC, not secure.
✓
Configure Cloud NAT and add firewall rules to allow only necessary egress and deny all ingress.
Why this is correct
Meets both outbound access and inbound blocking requirements.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Configure Cloud NAT and add a firewall rule to allow all egress traffic.
Why it's wrong here
Allows all egress but does not explicitly block inbound.
✗
Configure Cloud NAT and add a firewall rule to deny all ingress and egress.
Why it's wrong here
Denies outbound traffic, preventing updates.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume default firewall rules are sufficient for security, but they actually allow some ingress (e.g., ICMP from internal ranges), so a deny-all-ingress rule is necessary to fully block inbound traffic.
Detailed technical explanation
How to think about this question
Cloud NAT uses source network address translation (SNAT) to map private IPs to a single public IP for outbound connections, but it does not inspect or filter traffic; firewall rules are required for access control. Under the hood, Cloud NAT relies on the VPC's implicit egress rule (allow all outbound) unless overridden, so a deny-all-egress rule (as in Option D) would prevent NAT from working. In practice, you might allow only egress to specific destination IP ranges (e.g., update servers) using firewall rules with target tags and service accounts.
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.
Designing, planning, and prototyping a GCP network — This question tests Designing, planning, and prototyping a GCP network — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure Cloud NAT and add firewall rules to allow only necessary egress and deny all ingress. — Option B is correct because Cloud NAT provides outbound internet connectivity for instances without external IPs, and the explicit firewall rules ensure only necessary egress traffic is allowed while denying all ingress, meeting the requirement of no inbound traffic. Default firewall rules allow some ingress (e.g., ICMP), so they must be overridden with a deny-all-ingress rule to fully block inbound traffic.
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: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
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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