The correct answer is to create a single firewall rule with source ranges 10.0.1.0/24, 130.211.0.0/22, and 35.191.0.0/16, protocol tcp:22, and target tag 'ssh-allowed'. This works because a Google Cloud VPC firewall rule can include multiple source CIDR ranges in one rule, allowing you to combine the specific subnet access and the required health check ranges without needing separate rules. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of how to efficiently consolidate ingress rules using target tags and multiple source ranges, a common trap being the instinct to create separate rules for each source or to use overly broad allow-all policies. Remember that health check ranges are fixed and must be explicitly allowed alongside your internal subnets. A useful memory tip is "one rule, many sources" — think of it as a single gate that checks multiple IDs before letting SSH through.
PCNE Implementing network security 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. 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 wants to restrict SSH access to a VM instance to only a specific subnet (10.0.1.0/24) and allow all traffic from the health check ranges (130.211.0.0/22 and 35.191.0.0/16) for load balancing. Which firewall rule configuration should be used for the SSH rule?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create a rule with source ranges 10.0.1.0/24, 130.211.0.0/22, 35.191.0.0/16, protocol tcp:22, and target tag 'ssh-allowed'.
Option D is correct because a single firewall rule can specify both source ranges and target tags. Priority is not needed to be lowest unless conflicting. Option A is wrong because allowing all traffic is too broad. Option B is wrong because denying SSH would not allow health checks if not specified separately. Option C is wrong because it allows only health check ranges, not the subnet.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Create a rule allowing SSH from 0.0.0.0/0 and apply a tag to the VM.
Why it's wrong here
This allows SSH from anywhere, which is not secure.
✗
Create a rule allowing SSH from 10.0.1.0/24 and another rule allowing SSH from health check ranges.
Why it's wrong here
Health check ranges do not need SSH; they use HTTP/HTTPS.
✓
Create a rule with source ranges 10.0.1.0/24, 130.211.0.0/22, 35.191.0.0/16, protocol tcp:22, and target tag 'ssh-allowed'.
Why this is correct
This rule correctly restricts SSH to the subnet and health check ranges.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related PCNE subnetting questions on CIDR, address ranges, and subnet selection.
Implementing network security — This question tests Implementing network security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Create a rule with source ranges 10.0.1.0/24, 130.211.0.0/22, 35.191.0.0/16, protocol tcp:22, and target tag 'ssh-allowed'. — Option D is correct because a single firewall rule can specify both source ranges and target tags. Priority is not needed to be lowest unless conflicting. Option A is wrong because allowing all traffic is too broad. Option B is wrong because denying SSH would not allow health checks if not specified separately. Option C is wrong because it allows only health check ranges, not the subnet.
What should I do if I get this PCNE question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related PCNE subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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