- A
Ingress rule: target tag 'db', source tag 'app', allow tcp:3306, priority 1000.
This allows 'app' instances to connect to 'db' on 3306.
- B
Egress rule: target tag 'app', destination tag 'db', allow tcp:3306, priority 1000.
Why wrong: Egress rules control outbound traffic; this would allow 'app' to send to 'db', but does not control inbound to 'db'. It's unnecessary if ingress rule exists.
- C
Ingress rule: target tag 'db', source 0.0.0.0/0, deny tcp:3306, priority 2000.
Why wrong: This would deny all sources, including 'app', because deny with higher priority (lower number) would block 'app' if priority 2000 is lower than 1000? Actually priority 2000 is lower than 1000, so this deny with lower priority would be overridden by the allow; but the combination is messy. Better to have only the allow rule.
- D
Ingress rule: target tag 'db', source tag 'app', deny tcp:3306, priority 1000.
Why wrong: Denies the desired traffic.
- E
Ingress rule: target tag 'db', source 0.0.0.0/0, deny all ports, priority 500.
Why wrong: This default deny would block 'app' as well if priority 500 is higher than the allow's 1000. This would not work.
PCNE Implementing VPC Instances Practice Question
This PCNE practice question tests your understanding of implementing vpc instances. 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.
You need to allow instances with network tag 'db' in subnet-a to only accept connections on TCP port 3306 from instances with network tag 'app' in subnet-b. Which TWO firewall rules should you create? (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
Ingress rule: target tag 'db', source tag 'app', allow tcp:3306, priority 1000.
The correct approach is to create an ingress rule on subnet-a targeting 'db' tags, allowing TCP 3306 from source tag 'app' (which implies source instances with that tag). Alternatively, an egress rule on subnet-b could allow traffic to 'db' instances. However, the typical solution is ingress rule on the target. The options require two rules to ensure only 'app' instances can connect to 'db' and also block other sources.
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.
- ✓
Ingress rule: target tag 'db', source tag 'app', allow tcp:3306, priority 1000.
Why this is correct
This allows 'app' instances to connect to 'db' on 3306.
Related concept
CIDR notation defines the prefix length.
- ✗
Egress rule: target tag 'app', destination tag 'db', allow tcp:3306, priority 1000.
Why it's wrong here
Egress rules control outbound traffic; this would allow 'app' to send to 'db', but does not control inbound to 'db'. It's unnecessary if ingress rule exists.
- ✗
Ingress rule: target tag 'db', source 0.0.0.0/0, deny tcp:3306, priority 2000.
Why it's wrong here
This would deny all sources, including 'app', because deny with higher priority (lower number) would block 'app' if priority 2000 is lower than 1000? Actually priority 2000 is lower than 1000, so this deny with lower priority would be overridden by the allow; but the combination is messy. Better to have only the allow rule.
- ✗
Ingress rule: target tag 'db', source tag 'app', deny tcp:3306, priority 1000.
Why it's wrong here
Denies the desired traffic.
- ✗
Ingress rule: target tag 'db', source 0.0.0.0/0, deny all ports, priority 500.
Why it's wrong here
This default deny would block 'app' as well if priority 500 is higher than the allow's 1000. This would not work.
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 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.
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 PCNE subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this PCNE question test?
Implementing VPC Instances — This question tests Implementing VPC Instances — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Ingress rule: target tag 'db', source tag 'app', allow tcp:3306, priority 1000. — The correct approach is to create an ingress rule on subnet-a targeting 'db' tags, allowing TCP 3306 from source tag 'app' (which implies source instances with that tag). Alternatively, an egress rule on subnet-b could allow traffic to 'db' instances. However, the typical solution is ingress rule on the target. The options require two rules to ensure only 'app' instances can connect to 'db' and also block other sources.
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: Jul 4, 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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