The answer is yes, you need explicit firewall rules for inter-region VPC internal communication between instances in different subnets. This is because VPC firewall rules are stateful and apply globally across all regions within the same VPC, but the default implied deny-all ingress rule blocks all traffic unless a rule explicitly permits it. For the scenario with a VM at 10.0.1.2 in us-central1 and another at 10.0.2.2 in europe-west1, you must configure an ingress firewall rule on the destination VM’s subnet that allows traffic from the source subnet (10.0.1.0/24) or the specific source instance IP. On the Google Professional Cloud Network Engineer exam, this tests your understanding that VPCs are global resources, but firewall rules are not automatically permissive—a common trap is assuming subnets in the same VPC can communicate without rules. Remember the memory tip: “Same VPC, same firewall—rules are global, but ingress is not free.”
PCNE Implementing a Virtual Private Cloud Practice Question
This PCNE practice question tests your understanding of implementing a virtual private cloud. 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.
An organization has a VPC with custom mode subnets in us-central1 and europe-west1. They create a VM instance in us-central1 with an internal IP 10.0.1.2 and a VM in europe-west1 with internal IP 10.0.2.2. They want to enable communication between these instances using internal IPs. What must be configured?
Ensure the VPC firewall rules allow ingress from the source subnet or instance.
Firewall rules control traffic within a VPC; by default, all internal traffic is allowed, but custom rules could block it.
B
Set up VPC peering between the two regions.
Why wrong: VPC peering is used between separate VPCs, not within the same VPC.
C
No additional configuration is needed because internal IPs are routable within the VPC.
Why wrong: While internal IPs are routable, firewall rules must permit the traffic. Default rules allow it, but if custom rules are applied, they may block it.
D
Enable Cloud NAT for the VPC.
Why wrong: Cloud NAT is for internet access, not internal communication.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Ensure the VPC firewall rules allow ingress from the source subnet or instance.
Option A is correct because VPC firewall rules are stateful and must allow ingress traffic from the source subnet (10.0.1.0/24) or the specific source instance (10.0.1.2) to the destination VM in europe-west1. By default, VPCs have an implied deny-all ingress rule, so explicit firewall rules are required to permit traffic between subnets in different regions within the same VPC. The rule should specify the source IP range or tag and the destination protocol/port (e.g., ICMP, TCP/22) to enable communication.
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.
✓
Ensure the VPC firewall rules allow ingress from the source subnet or instance.
Why this is correct
Firewall rules control traffic within a VPC; by default, all internal traffic is allowed, but custom rules could block it.
Related concept
Read the scenario before looking for a memorised answer.
✗
Set up VPC peering between the two regions.
Why it's wrong here
VPC peering is used between separate VPCs, not within the same VPC.
✗
No additional configuration is needed because internal IPs are routable within the VPC.
Why it's wrong here
While internal IPs are routable, firewall rules must permit the traffic. Default rules allow it, but if custom rules are applied, they may block it.
✗
Enable Cloud NAT for the VPC.
Why it's wrong here
Cloud NAT is for internet access, not internal communication.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that internal IPs are automatically reachable across regions within the same VPC, but the trap is that while routing is global by default, firewall rules are not — candidates forget that an explicit ingress rule is required to allow cross-subnet traffic.
Detailed technical explanation
How to think about this question
Under the hood, Google Cloud VPCs use a global routing table that automatically routes traffic between all subnets in the same VPC, regardless of region, using Google's internal backbone. However, the firewall is a distributed stateful service that evaluates each packet against ingress and egress rules; by default, an implied deny ingress rule blocks all traffic from other subnets, so a custom ingress rule must be created to allow the specific source (e.g., 10.0.1.0/24) to reach the destination. A real-world scenario is a multi-tier application where a web server in us-central1 needs to query a database in europe-west1; without an ingress firewall rule allowing TCP/3306 from the web subnet, the connection will fail even though routing works.
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.
Implementing a Virtual Private Cloud — This question tests Implementing a Virtual Private Cloud — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Ensure the VPC firewall rules allow ingress from the source subnet or instance. — Option A is correct because VPC firewall rules are stateful and must allow ingress traffic from the source subnet (10.0.1.0/24) or the specific source instance (10.0.1.2) to the destination VM in europe-west1. By default, VPCs have an implied deny-all ingress rule, so explicit firewall rules are required to permit traffic between subnets in different regions within the same VPC. The rule should specify the source IP range or tag and the destination protocol/port (e.g., ICMP, TCP/22) to enable communication.
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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