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Question 838 of 982

Implementing VPC Instances →hardMultiple ChoiceObjective-mapped

PCNE Implementing VPC Instances Practice Question

This PCNE practice question tests your understanding of implementing vpc instances. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 a subnet in us-central1. Compute Engine instances in that subnet have no external IPs but need to reach the internet for software updates. The engineer configured Cloud NAT with the default settings. However, instances fail to reach the internet. 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.

A
Cloud NAT is not configured in the correct region.
Why wrong: The NAT gateway must be in the same region as the instances. The scenario mentions us-central1, which should be fine.
B
An egress firewall rule is missing that allows traffic from the instances to the internet via NAT.
Firewall rules control egress traffic; Cloud NAT does not bypass them. A rule allowing egress to 0.0.0.0/0 is needed.
C
Private Google Access is enabled on the subnet, which overrides Cloud NAT.
Why wrong: Private Google Access does not affect general internet access via NAT.
D
The Cloud NAT router is not attached to the correct VPC network.
Why wrong: If it were not attached, NAT wouldn't work at all. The issue is likely firewall-related.

Full breakdown with real-world context →

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 egress firewall rule is missing that allows traffic from the instances to the internet via NAT.

Cloud NAT requires that the instances are allowed egress traffic to the internet. Without a suitable egress firewall rule, NAT traffic is blocked. The default firewall rules only allow egress to Google APIs (private.googleapis.com) but not to general internet destinations. A firewall rule allowing egress to 0.0.0.0/0 on the appropriate ports is required.

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.

✗
Cloud NAT is not configured in the correct region.
Why it's wrong here
The NAT gateway must be in the same region as the instances. The scenario mentions us-central1, which should be fine.
✓
An egress firewall rule is missing that allows traffic from the instances to the internet via NAT.
Why this is correct
Firewall rules control egress traffic; Cloud NAT does not bypass them. A rule allowing egress to 0.0.0.0/0 is needed.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
Private Google Access is enabled on the subnet, which overrides Cloud NAT.
Why it's wrong here
Private Google Access does not affect general internet access via NAT.
✗
The Cloud NAT router is not attached to the correct VPC network.
Why it's wrong here
If it were not attached, NAT wouldn't work at all. The issue is likely firewall-related.

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.

Trap categories for this question

Scenario analysis trap
The NAT gateway must be in the same region as the instances. The scenario mentions us-central1, which should be fine.

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 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: An egress firewall rule is missing that allows traffic from the instances to the internet via NAT. — Cloud NAT requires that the instances are allowed egress traffic to the internet. Without a suitable egress firewall rule, NAT traffic is blocked. The default firewall rules only allow egress to Google APIs (private.googleapis.com) but not to general internet destinations. A firewall rule allowing egress to 0.0.0.0/0 on the appropriate ports is required.

What should I do if I get this PCNE question wrong?

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: Jul 4, 2026

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