- A
A deny rule with a higher priority (lower number) exists and blocks the traffic.
A deny rule with lower priority number overrides allow rules of higher priority number.
- B
The firewall rule is in the wrong network.
Why wrong: The rule is in the same network as the instance, otherwise the instance would not be reachable at all.
- C
The instance has an internal IP address that is not within the source range of the rule.
Why wrong: The source range applies to the source of the traffic, not the instance's IP.
- D
The firewall rule's target tag is misspelled.
Why wrong: The problem states the target tag is set correctly.
Question 75 of 1,000
easyMultiple ChoiceObjective-mapped
Why a VPC Firewall Allow Rule May Not Work: Priority Deny Rule
This PCSE practice question tests your understanding of pcse exam topics. 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 security engineer is troubleshooting a VPC firewall rule that is not allowing traffic from a specific subnet to a Compute Engine instance. The target tag is set correctly. 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.
Quick Answer
The answer is a deny rule with a higher priority (lower number) that blocks the traffic. This is because VPC firewall rules are evaluated in order of priority, where a lower numerical value indicates higher precedence; if a deny rule has a priority of 1000 and an allow rule has a priority of 2000, the deny rule will be matched and enforced first, overriding the allow rule regardless of the target tag or source range. On the Google Professional Cloud Security Engineer exam, this concept tests your understanding of VPC firewall rule priority and the implicit deny at the end of the rule hierarchy—a common trap is assuming allow rules automatically override denies, but the priority number dictates the outcome. Remember the mnemonic: lower number, higher power; a deny with a lower number will always overpower an allow with a higher number.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A deny rule with a higher priority (lower number) exists and blocks the traffic.
Option A is correct because if a deny rule with a lower priority number (higher priority) exists, it will take precedence and block traffic despite an allow rule. Option B is incorrect because the instance's internal IP address does not affect the source range check. Option C is incorrect because the problem states the target tag is set correctly. Option D is incorrect because the rule is in the correct network.
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.
- ✓
A deny rule with a higher priority (lower number) exists and blocks the traffic.
Why this is correct
A deny rule with lower priority number overrides allow rules of higher priority number.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
- ✗
The firewall rule is in the wrong network.
Why it's wrong here
The rule is in the same network as the instance, otherwise the instance would not be reachable at all.
- ✗
The instance has an internal IP address that is not within the source range of the rule.
Why it's wrong here
The source range applies to the source of the traffic, not the instance's IP.
- ✗
The firewall rule's target tag is misspelled.
Why it's wrong here
The problem states the target tag is set correctly.
Common exam traps
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
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.
Visual reference
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 PCSE subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this PCSE question test?
CIDR notation defines the prefix length.
What is the correct answer to this question?
The correct answer is: A deny rule with a higher priority (lower number) exists and blocks the traffic. — Option A is correct because if a deny rule with a lower priority number (higher priority) exists, it will take precedence and block traffic despite an allow rule. Option B is incorrect because the instance's internal IP address does not affect the source range check. Option C is incorrect because the problem states the target tag is set correctly. Option D is incorrect because the rule is in the correct network.
What should I do if I get this PCSE 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 PCSE 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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Last reviewed: Jun 24, 2026
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