The answer is that the firewall rule must be placed in the destination VPC, not the source VPC. This is because Google Cloud firewall rules are applied at the network level to control inbound traffic to the instances within that network. For SSH traffic from VPC-A to reach instances in VPC-B, the rule allowing ingress on TCP port 22 from source range 10.0.0.0/16 must exist in VPC-B, as it governs what traffic is permitted to enter that destination network. On the Google Professional Cloud Network Engineer exam, this concept frequently appears as a trap where candidates instinctively create the rule in the source VPC, mirroring on-premises thinking. The key distinction is that GCP firewalls are stateful and destination-centric—they evaluate traffic as it arrives at the target instance’s network interface. A simple memory tip: “Firewalls guard the door you walk into, not the door you walk out of.”
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.
A company has two VPCs in the same project: VPC-A (10.0.0.0/16) and VPC-B (10.1.0.0/16). They want to allow SSH from VPC-A to instances in VPC-B. The network admin creates a firewall rule with source range 10.0.0.0/16 and protocol tcp:22, but connectivity fails. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The rule is created in VPC-A instead of VPC-B.
Firewall rules in Google Cloud are applied at the network level, not the instance level. To allow inbound SSH traffic from VPC-A to instances in VPC-B, the firewall rule must be created in VPC-B (the destination network) with the source range set to 10.0.0.0/16. Creating the rule in VPC-A would only affect traffic destined for instances in VPC-A, not VPC-B.
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 rule is created in VPC-A instead of VPC-B.
Why this is correct
Firewall rules apply to the VPC they are created in. An ingress rule for VPC-B must be created in VPC-B.
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 source range should be 10.0.0.0/8.
Why it's wrong here
The source range is already correct for VPC-A.
✗
The target tag is not specified.
Why it's wrong here
Target tags are optional; if not specified, the rule applies to all instances.
✗
The source tag is missing.
Why it's wrong here
Tags are not required when using source ranges.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that firewall rules are applied at the source network or that rules are global across VPCs, when in fact each VPC has its own independent set of firewall rules and ingress rules must be in the destination VPC.
Detailed technical explanation
How to think about this question
In Google Cloud VPC, firewall rules are stateful and evaluated at the network edge. Each rule is associated with a specific VPC network and direction (ingress/egress). For ingress rules, the source can be defined by CIDR ranges, tags, or service accounts. The rule must be in the destination VPC (VPC-B) to allow inbound traffic; a rule in VPC-A only controls traffic entering VPC-A. This is analogous to how on-premises firewalls require rules on the destination interface.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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 rule is created in VPC-A instead of VPC-B. — Firewall rules in Google Cloud are applied at the network level, not the instance level. To allow inbound SSH traffic from VPC-A to instances in VPC-B, the firewall rule must be created in VPC-B (the destination network) with the source range set to 10.0.0.0/16. Creating the rule in VPC-A would only affect traffic destined for instances in VPC-A, not VPC-B.
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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