The answer is that on-premises DNS is not configured to resolve Google API hostnames to the Private Google Access IP address range 199.36.153.4/30. This is the root cause because Private Google Access for on-premises relies on a specific, non-public IP range that must be returned by your on-premises DNS servers. When DNS resolves these hostnames to their standard public IP addresses instead, on-premises hosts follow their default internet route, bypassing the Dedicated Interconnect entirely—even though the interconnect is available and properly configured. On the Google Professional Cloud Network Engineer exam, this question tests your understanding of how DNS resolution directly controls traffic routing in hybrid networks. A common trap is assuming that simply having an interconnect is enough; the exam wants you to know that DNS must explicitly point to 199.36.153.4/30 for the private access path to be used. Memory tip: think of it as “DNS drives the route”—if DNS says public, traffic goes public; if DNS says 199.36.153.4/30, traffic stays private over the interconnect.
PCNE Implementing network security Practice Question
This PCNE practice question tests your understanding of implementing network security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 hybrid network with on-premises data center connected to Google Cloud via Dedicated Interconnect. They use Private Google Access for on-premises (on-premises hosts use the external IP addresses of Google APIs via the interconnect). However, they notice that traffic to certain Google APIs is being routed via the internet instead of the interconnect. What is a likely cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
On-premises DNS is not configured to resolve Google API hostnames to the Private Google Access IP address range (199.36.153.4/30).
Private Google Access for on-premises requires on-premises DNS to resolve Google API hostnames to the specific IP range 199.36.153.4/30. If DNS resolution returns the public IP addresses instead, traffic will be routed over the internet rather than through the Dedicated Interconnect, even though the interconnect is available. This is because the on-premises hosts will use the public IPs and follow their default route to the internet.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
On-premises DNS is not configured to resolve Google API hostnames to the Private Google Access IP address range (199.36.153.4/30).
Read the scenario before looking for a memorised answer.
✗
Firewall rules in the VPC are blocking the private API traffic.
Why it's wrong here
Firewall rules control traffic within VPC, not routing decisions.
✗
Cloud NAT is not configured for the on-premises subnet.
Why it's wrong here
Cloud NAT is for VMs without external IPs, not for on-premises.
✗
VPC Flow Logs are not enabled, causing routing misconfiguration.
Why it's wrong here
Flow logs do not affect routing.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that firewall rules or Cloud NAT are the primary cause of routing failures in hybrid connectivity, when the real issue is DNS resolution not returning the correct private IP range for Private Google Access.
Detailed technical explanation
How to think about this question
Private Google Access for on-premises relies on DNS resolution to the 199.36.153.4/30 range, which is a special Google-owned IP block that is advertised over the interconnect via BGP. The on-premises router must have a route for this prefix pointing to the interconnect, and the DNS server must return these IPs for the API hostnames. If either DNS or routing is misconfigured, traffic will fall back to the internet. This is distinct from Private Google Access for VPC subnets, which uses RFC 1918 addresses and VPC private DNS zones.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Implementing network security — This question tests Implementing network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: On-premises DNS is not configured to resolve Google API hostnames to the Private Google Access IP address range (199.36.153.4/30). — Private Google Access for on-premises requires on-premises DNS to resolve Google API hostnames to the specific IP range 199.36.153.4/30. If DNS resolution returns the public IP addresses instead, traffic will be routed over the internet rather than through the Dedicated Interconnect, even though the interconnect is available. This is because the on-premises hosts will use the public IPs and follow their default route to the internet.
What should I do if I get this PCNE question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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