The answer is a custom static route for 199.153.4/30 pointing to the interconnect, which overrides the proxy’s default internet gateway egress. Private Google Access allows instances with only internal IPs to reach Google APIs via the default route (0.0.0.0/0) to the internet gateway, but it does not create or modify routes. When a forward proxy in us-central1 sends outbound traffic to Google API VIPs, the VPC routing table applies the most specific match—if a custom route for 199.153.4/30 exists and points to the interconnect, the proxy’s traffic is forced back on-premises instead of out the internet gateway, breaking access for on-premises users who rely on that proxy. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding that PGA enables API access for internal instances, but custom routes for the Google API IP range can hijack that traffic, a common trap when mixing interconnect with proxy architectures. Remember: PGA gives permission, but routes control direction—a specific route to on-premises beats a default route to the internet every time.
PCNE Implementing a Virtual Private Cloud Practice Question
This PCNE practice question tests your understanding of implementing a virtual private cloud. 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 company has an on-premises data center connected to Google Cloud via Dedicated Interconnect. They have a VPC with subnets in us-central1 and us-west1. They want compute instances in us-central1 to access Google APIs (e.g., Cloud Storage) without traversing the internet, but the on-premises network must also be able to access those APIs via the interconnect. They have configured Private Google Access (PGA) on all subnets. However, on-premises users report that they cannot access Cloud Storage buckets using the private IP of a forward proxy in us-central1 (the proxy is configured to use the default internet gateway for egress). What is the most likely reason?
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.
Private Google Access is not supported on subnets in us-central1.
Why wrong: PGA is supported on all subnets.
B
The forward proxy must use an external IP address to use Private Google Access.
Why wrong: PGA is designed for instances without external IPs.
C
There is a custom static route for 199.36.153.4/30 (Google API VIP) that points to the interconnect, overriding the default route for the proxy's outbound traffic.
A custom route for the Google API VIP would cause the proxy to route traffic to on-premises instead of using the internet gateway, breaking PGA for the proxy.
D
The on-premises network must be configured with a default route pointing to the internet.
Why wrong: On-premises routing is irrelevant; the issue is on the proxy's VPC routing.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
There is a custom static route for 199.36.153.4/30 (Google API VIP) that points to the interconnect, overriding the default route for the proxy's outbound traffic.
Private Google Access allows instances with only internal IPs to reach Google APIs via the default internet gateway. However, on-premises traffic coming via interconnect uses the VPC's internal IP range, and if the forward proxy does not have a route for Google API destinations via the internet gateway (default route), it will try to use the interconnect route, which points to on-premises. Since the proxy is configured to use the default internet gateway, but that gateway is only effective for instances with PGA; on-premises traffic does not go through the proxy's default gateway. The issue is that the proxy's egress traffic to Google APIs is being routed via the on-premises network because the VPC's default route (0.0.0.0/0) points to the internet gateway only for instances with PGA, but for traffic sourced from the proxy that is destined to Google APIs, the proxy itself uses its default gateway which is the internet gateway. Actually, the on-premises users are using the proxy's internal IP as a forward proxy. The proxy will make requests to Google APIs. For those requests, the proxy's VPC will route based on the most specific route. If there is a custom route for the Google API IP ranges (e.g., 199.36.153.4/30) that points to the interconnect, the proxy will send traffic to on-premises instead of internet. PGA does not create routes; it only allows the default route to be used for Google API destinations. A common misconfiguration is having a custom route for the Google API IP range (e.g., from a previous VPN setup) that overrides the default route. Option B is correct.
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.
✗
Private Google Access is not supported on subnets in us-central1.
Why it's wrong here
PGA is supported on all subnets.
✗
The forward proxy must use an external IP address to use Private Google Access.
Why it's wrong here
PGA is designed for instances without external IPs.
✓
There is a custom static route for 199.36.153.4/30 (Google API VIP) that points to the interconnect, overriding the default route for the proxy's outbound traffic.
Why this is correct
A custom route for the Google API VIP would cause the proxy to route traffic to on-premises instead of using the internet gateway, breaking PGA for the proxy.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The on-premises network must be configured with a default route pointing to the internet.
Why it's wrong here
On-premises routing is irrelevant; the issue is on the proxy's VPC routing.
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.
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: There is a custom static route for 199.36.153.4/30 (Google API VIP) that points to the interconnect, overriding the default route for the proxy's outbound traffic. — Private Google Access allows instances with only internal IPs to reach Google APIs via the default internet gateway. However, on-premises traffic coming via interconnect uses the VPC's internal IP range, and if the forward proxy does not have a route for Google API destinations via the internet gateway (default route), it will try to use the interconnect route, which points to on-premises. Since the proxy is configured to use the default internet gateway, but that gateway is only effective for instances with PGA; on-premises traffic does not go through the proxy's default gateway. The issue is that the proxy's egress traffic to Google APIs is being routed via the on-premises network because the VPC's default route (0.0.0.0/0) points to the internet gateway only for instances with PGA, but for traffic sourced from the proxy that is destined to Google APIs, the proxy itself uses its default gateway which is the internet gateway. Actually, the on-premises users are using the proxy's internal IP as a forward proxy. The proxy will make requests to Google APIs. For those requests, the proxy's VPC will route based on the most specific route. If there is a custom route for the Google API IP ranges (e.g., 199.36.153.4/30) that points to the interconnect, the proxy will send traffic to on-premises instead of internet. PGA does not create routes; it only allows the default route to be used for Google API destinations. A common misconfiguration is having a custom route for the Google API IP range (e.g., from a previous VPN setup) that overrides the default route. Option B is correct.
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.
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