- A
The instance does not have a DNS resolution for storage.googleapis.com.
Why wrong: DNS resolution would still work even without Private Google Access; the issue is connectivity.
- B
The subnet's Private Google Access is enabled, but Cloud NAT is also routing traffic to Google APIs via NAT, bypassing the internal access.
When Cloud NAT is enabled, the default route (0.0.0.0/0) points to NAT, which may override Private Google Access routes. Private Google Access requires a specific route for 199.36.153.4/30 with next hop 'default internet gateway'.
- C
The VPC does not have a default route to the internet.
Why wrong: Private Google Access does not require a default internet route; it uses Google's private backbone.
- D
The instance does not have a firewall rule allowing egress to 199.36.153.4/30.
Why wrong: Private Google Access uses 199.36.153.4/30 for restricted APIs, but if the issue is with storage.googleapis.com, it might be using the public IP range.
Quick Answer
The answer is that Cloud NAT is overriding Private Google Access by routing traffic to Google APIs through the NAT gateway instead of using the internal default route. This happens because when Cloud NAT is configured on a subnet, it creates a default route (0.0.0.0/0) with a next hop of the NAT gateway, which takes precedence over the Private Google Access route that uses the default internet gateway for the restricted Google API VIPs. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of route priority and how Private Google Access relies on specific routes to 199.36.153.4/30 and 199.36.153.8/30—a common trap is assuming both features can coexist without conflict. The key insight is that Cloud NAT’s broader default route will match and redirect all outbound traffic, including Google API destinations, unless you explicitly exclude those ranges from NAT. Memory tip: think of Private Google Access as a “private shortcut” that Cloud NAT’s “public detour” can accidentally override.
PCNE Implementing network security Practice Question
This PCNE practice question tests your understanding of implementing network security. 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 GCP environment has a VPC with a subnet that enables Private Google Access. Instances in that subnet can access Google APIs without external IPs. However, an instance cannot reach storage.googleapis.com from a private IP. Cloud NAT is configured for the subnet. What is the most likely reason for the failure?
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.
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
The subnet's Private Google Access is enabled, but Cloud NAT is also routing traffic to Google APIs via NAT, bypassing the internal access.
Private Google Access uses the default routes for Google APIs. If Cloud NAT is configured, it may override Private Google Access because traffic to Google APIs can be matched by a default route with a next hop of NAT. To fix, ensure the subnet has a Private Google Access route with a next hop 'default internet gateway' for the restricted VIPs. Option B correctly identifies that Cloud NAT is incorrectly routing traffic.
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.
- ✗
The instance does not have a DNS resolution for storage.googleapis.com.
Why it's wrong here
DNS resolution would still work even without Private Google Access; the issue is connectivity.
- ✓
The subnet's Private Google Access is enabled, but Cloud NAT is also routing traffic to Google APIs via NAT, bypassing the internal access.
Why this is correct
When Cloud NAT is enabled, the default route (0.0.0.0/0) points to NAT, which may override Private Google Access routes. Private Google Access requires a specific route for 199.36.153.4/30 with next hop 'default internet gateway'.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
- ✗
The VPC does not have a default route to the internet.
Why it's wrong here
Private Google Access does not require a default internet route; it uses Google's private backbone.
- ✗
The instance does not have a firewall rule allowing egress to 199.36.153.4/30.
Why it's wrong here
Private Google Access uses 199.36.153.4/30 for restricted APIs, but if the issue is with storage.googleapis.com, it might be using the public IP range.
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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FAQ
Questions learners often ask
What does this PCNE question test?
Implementing network security — This question tests Implementing network security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The subnet's Private Google Access is enabled, but Cloud NAT is also routing traffic to Google APIs via NAT, bypassing the internal access. — Private Google Access uses the default routes for Google APIs. If Cloud NAT is configured, it may override Private Google Access because traffic to Google APIs can be matched by a default route with a next hop of NAT. To fix, ensure the subnet has a Private Google Access route with a next hop 'default internet gateway' for the restricted VIPs. Option B correctly identifies that Cloud NAT is incorrectly routing traffic.
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.
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.
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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