- A
An ingress rule on the db tier instances with tag 'db' allowing TCP 3306 from instances with tag 'app'.
This ensures only app tier can reach db tier on port 3306.
- B
An egress rule on the web instances with tag 'web' allowing TCP 8080 to the app subnet.
Why wrong: Egress rules control outbound traffic, but the requirement is to control ingress into app and db tiers; ingress rules are more direct.
- C
An egress rule on the web subnet allowing TCP 8080 to any destination.
Why wrong: This does not restrict which tier can communicate; it allows all egress on port 8080, not just to app.
- D
An ingress rule on the app tier instances with tag 'app' allowing TCP 8080 from instances with tag 'web'.
This ensures only web tier can reach app tier on port 8080.
- E
An ingress rule on the web subnet allowing TCP 8080 from the app subnet.
Why wrong: This allows app to communicate with web on 8080, which is the opposite direction.
Quick Answer
The correct answer is to create an ingress rule on the app tier instances with tag 'app' allowing TCP 8080 from instances with tag 'web', and an ingress rule on the db tier instances with tag 'db' allowing TCP 3306 from instances with tag 'app'. This approach works because Google Cloud firewall rules are stateful and evaluated at the instance level using source and destination tags, meaning you control access by defining what traffic is allowed to enter the target tier rather than by restricting outbound traffic from the source. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of multi-tier VPC firewall rules using tags, a common pattern for microsegmentation where each tier only listens for traffic from the preceding tier. A frequent trap is choosing egress rules on the source tier, which fail to block other unauthorized sources from reaching the destination; always remember that ingress rules on the destination are the correct enforcement point. Memory tip: “Ingress on the listener, tag the source” — the rule goes on the tier receiving traffic, and the source tag identifies who is allowed to knock.
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.
A company has a VPC with three tiers: web, app, and db. They want to enforce that only the web tier can communicate with the app tier on TCP port 8080, and only the app tier can communicate with the db tier on TCP port 3306. All instances are in the same region but different subnets. Which TWO firewall rules should be created? (Choose 2.)
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
An ingress rule on the db tier instances with tag 'db' allowing TCP 3306 from instances with tag 'app'.
To control traffic between tiers, you use firewall rules with source tags. The correct approach is to create ingress rules on the destination tier's subnet, allowing traffic from the source tier's tag. Option B is an egress rule on the web tier to allow to app tier on 8080, but that doesn't prevent other sources from reaching app tier. Option D is an ingress rule on app tier allowing from web tag on 8080. Option E is an ingress rule on db tier allowing from app tag on 3306. Option A and C are incorrect because they allow traffic from any source or are on the wrong tier.
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.
- ✓
An ingress rule on the db tier instances with tag 'db' allowing TCP 3306 from instances with tag 'app'.
Why this is correct
This ensures only app tier can reach db tier on port 3306.
Related concept
CIDR notation defines the prefix length.
- ✗
An egress rule on the web instances with tag 'web' allowing TCP 8080 to the app subnet.
Why it's wrong here
Egress rules control outbound traffic, but the requirement is to control ingress into app and db tiers; ingress rules are more direct.
- ✗
An egress rule on the web subnet allowing TCP 8080 to any destination.
Why it's wrong here
This does not restrict which tier can communicate; it allows all egress on port 8080, not just to app.
- ✓
An ingress rule on the app tier instances with tag 'app' allowing TCP 8080 from instances with tag 'web'.
Why this is correct
This ensures only web tier can reach app tier on port 8080.
Related concept
CIDR notation defines the prefix length.
- ✗
An ingress rule on the web subnet allowing TCP 8080 from the app subnet.
Why it's wrong here
This allows app to communicate with web on 8080, which is the opposite direction.
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
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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.
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Implementing a Virtual Private Cloud — study guide chapter
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FAQ
Questions learners often ask
What does this PCNE question test?
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: An ingress rule on the db tier instances with tag 'db' allowing TCP 3306 from instances with tag 'app'. — To control traffic between tiers, you use firewall rules with source tags. The correct approach is to create ingress rules on the destination tier's subnet, allowing traffic from the source tier's tag. Option B is an egress rule on the web tier to allow to app tier on 8080, but that doesn't prevent other sources from reaching app tier. Option D is an ingress rule on app tier allowing from web tag on 8080. Option E is an ingress rule on db tier allowing from app tag on 3306. Option A and C are incorrect because they allow traffic from any source or are on the wrong tier.
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.
About these practice questions
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Last reviewed: Jun 24, 2026
This PCNE practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCNE exam.
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