- A
An ingress rule that allows all traffic from the subnet to the instances (target tags or service account)
This allows internal communication within the subnet.
- B
An egress rule that blocks all traffic except to the load balancer and subnet
Why wrong: The default egress rule allows all outbound traffic; you would need a deny rule with lower priority, but the question asks for rules to allow, not block.
- C
An ingress rule that allows SSH (tcp:22) from the bastion host (using its service account or tags)
This allows administrative access from the bastion.
- D
An egress rule that allows all traffic to the load balancer's frontend IP
Why wrong: Egress rules are not required for inbound traffic; the load balancer's health checks may need ingress rules, but not egress.
- E
An ingress rule that allows HTTP traffic from the load balancer to the instances
Why wrong: If the load balancer is internal, it will use the subnet IP range; the first rule (A) already covers that. If external, you need a specific rule for the load balancer's health check ranges.
PCSE Configuring Network Security Practice Question
This PCSE practice question tests your understanding of configuring network security. 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 is deploying a new internal application on Google Cloud. They want to ensure that VM instances in a specific subnet can only communicate with each other and with a load balancer that fronts the application. They also want to allow SSH access from a bastion host. Which TWO firewall rules should they create? (Choose two.)
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 that allows all traffic from the subnet to the instances (target tags or service account)
To restrict communication to only internal subnet traffic and the load balancer, you need an ingress rule that allows traffic from the subnet (source) to all instances (target). To allow SSH from the bastion, you need an ingress rule allowing TCP port 22 from the bastion host (using its service account or tags). Egress rules are not needed because the default allow egress is not restrictive; but if you want to block other egress, you would need a deny egress rule, but the question asks for rules to allow, not block.
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 that allows all traffic from the subnet to the instances (target tags or service account)
Why this is correct
This allows internal communication within the subnet.
Related concept
CIDR notation defines the prefix length.
- ✗
An egress rule that blocks all traffic except to the load balancer and subnet
Why it's wrong here
The default egress rule allows all outbound traffic; you would need a deny rule with lower priority, but the question asks for rules to allow, not block.
- ✓
An ingress rule that allows SSH (tcp:22) from the bastion host (using its service account or tags)
Why this is correct
This allows administrative access from the bastion.
Related concept
CIDR notation defines the prefix length.
- ✗
An egress rule that allows all traffic to the load balancer's frontend IP
Why it's wrong here
Egress rules are not required for inbound traffic; the load balancer's health checks may need ingress rules, but not egress.
- ✗
An ingress rule that allows HTTP traffic from the load balancer to the instances
Why it's wrong here
If the load balancer is internal, it will use the subnet IP range; the first rule (A) already covers that. If external, you need a specific rule for the load balancer's health check ranges.
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 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.
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?
Configuring Network Security — This question tests Configuring Network Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: An ingress rule that allows all traffic from the subnet to the instances (target tags or service account) — To restrict communication to only internal subnet traffic and the load balancer, you need an ingress rule that allows traffic from the subnet (source) to all instances (target). To allow SSH from the bastion, you need an ingress rule allowing TCP port 22 from the bastion host (using its service account or tags). Egress rules are not needed because the default allow egress is not restrictive; but if you want to block other egress, you would need a deny egress rule, but the question asks for rules to allow, not block.
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.
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 PCSE 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 PCSE exam.
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