The answer is that the firewall rules have a source or destination filter that excludes the new subnet's CIDR. This is the most likely reason because Google Cloud firewall rules are stateful and can combine target tags with IP range filters; if a rule specifies a source or destination range that does not include 192.168.1.0/24, the rule will not apply to instances in that subnet, even if the target tags match. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding that firewall rules are scoped to the entire VPC, not individual subnets, and that tags alone do not override explicit IP range restrictions—a common trap is assuming tags guarantee rule application across all subnets. Remember the memory tip: "Tags target the instance, but IP ranges target the traffic; both must align for the rule to fire."
PCNE Implementing a Virtual Private Cloud Practice Question
This PCNE practice question tests your understanding of implementing a virtual private cloud. 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.
An organization uses a custom mode VPC with several subnets. They need to add a new subnet 192.168.1.0/24 for a new workload. After creating the subnet, they find that existing firewall rules with target tags don't apply to instances in the new subnet, even though the tags are applied. What is the most likely reason?
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 firewall rules have a source or destination filter that excludes the new subnet's CIDR.
Option C is correct because firewall rules may include source or destination IP range filters. Even if tags match, if the rule also specifies a source or destination IP range that does not include 192.168.1.0/24, the rule will not apply. Option A is incorrect because firewall rules are scoped to the VPC, not to subnets, so tags work across subnets. Option B is incorrect because there is no such concept as firewall rule enforcement on subnets. Option D is incorrect because instances do not need to be recreated; changes to firewall rules apply immediately.
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.
✗
Firewall rules are scoped to a VPC, not to subnets.
Why it's wrong here
That is true but does not cause the issue; tags work across subnets within the same VPC.
✗
The new subnet was created without enabling firewall rule enforcement.
Why it's wrong here
There is no such setting in Google Cloud.
✗
The instances need to be recreated for the new rules to apply.
Why it's wrong here
Firewall rules apply to existing instances without restart.
✓
The firewall rules have a source or destination filter that excludes the new subnet's CIDR.
Why this is correct
If a rule includes an IP range filter, it will only apply to traffic that matches that range.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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 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.
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 a Virtual Private Cloud — This question tests Implementing a Virtual Private Cloud — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The firewall rules have a source or destination filter that excludes the new subnet's CIDR. — Option C is correct because firewall rules may include source or destination IP range filters. Even if tags match, if the rule also specifies a source or destination IP range that does not include 192.168.1.0/24, the rule will not apply. Option A is incorrect because firewall rules are scoped to the VPC, not to subnets, so tags work across subnets. Option B is incorrect because there is no such concept as firewall rule enforcement on subnets. Option D is incorrect because instances do not need to be recreated; changes to firewall rules apply immediately.
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.
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?
CIDR notation defines the prefix length.
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