The correct answer is an ingress firewall rule with source 203.0.113.5/32, destination 10.0.1.2/32, and TCP port 22. This is because Google Cloud firewall rules are stateful and applied at the instance network interface level, not on the VPN tunnel itself; the Cloud VPN and Cloud Router handle routing via BGP, but the firewall explicitly controls which traffic is permitted to reach the instance. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding that firewall rules must reference the actual on-premises source IP, not the VPN gateway IP, and that destination must be the VM’s internal IP—a common trap is assuming the rule applies to the tunnel interface. Remember the key distinction: routing gets traffic to the VPC, but firewalls decide if it can enter the instance. Memory tip: “Route the packet, then firewall the port.”
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. 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 has a VPC with subnets in us-east1 and europe-west1. They have a Compute Engine instance in us-east1 with an internal IP 10.0.1.2. They need to allow SSH (port 22) from a specific on-premises IP 203.0.113.5 via Cloud VPN. The Cloud VPN tunnel uses a Cloud Router with BGP. The on-premises network advertises the route for 203.0.113.5/32 to the Cloud Router. Which firewall rule must be created?
Option D is correct because firewall rules in GCP are stateful and applied at the instance level, not the VPN tunnel. The rule must allow ingress traffic from the on-premises source IP (203.0.113.5/32) to the Compute Engine instance's internal IP (10.0.1.2/32) on TCP port 22. The Cloud VPN and Cloud Router handle routing, but the firewall rule explicitly defines the allowed traffic flow.
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.
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the direction of traffic in firewall rules, mistakenly thinking the on-premises IP should be the destination (as in Option B) or that the VPN tunnel's link-local address is the correct destination (as in Option A), when in fact the rule must match the actual source and destination IPs of the SSH session.
Detailed technical explanation
How to think about this question
GCP firewall rules are stateful, meaning that if an ingress rule allows traffic, the corresponding egress response is automatically permitted. The Cloud Router with BGP dynamically learns the on-premises route for 203.0.113.5/32, but firewall rules are independent of routing and must explicitly match the source and destination IPs. In a real-world scenario, if the on-premises network uses NAT, the source IP in the firewall rule should match the public IP after NAT, not the private IP of the host.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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: Ingress rule: source 203.0.113.5/32, destination 10.0.1.2/32, tcp:22 — Option D is correct because firewall rules in GCP are stateful and applied at the instance level, not the VPN tunnel. The rule must allow ingress traffic from the on-premises source IP (203.0.113.5/32) to the Compute Engine instance's internal IP (10.0.1.2/32) on TCP port 22. The Cloud VPN and Cloud Router handle routing, but the firewall rule explicitly defines the allowed traffic flow.
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.
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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