Resolving IP Range Overlap on GCP for the PCA Exam

IP range overlap is one of those topics that keeps showing up in the Professional Cloud Architect exam from different angles. It comes up when you merge two VPCs, when you bridge networks with Cloud VPN, and when you connect on-prem to GCP through Cloud Interconnect. The underlying problem is the same in all three cases, but the framing on the exam is different enough that I want to walk through each one in a single place.

Why Overlapping IP Ranges Break Networks

Every device on a network needs an address that uniquely identifies it within the routing context it lives in. When two networks that need to talk to each other use the same private IP range, routers cannot tell which network a given packet is destined for. Packets get misrouted, dropped, or sent to the wrong host. Direct integration is impossible until the conflict is resolved.

This is a private addressing problem. RFC 1918 ranges like 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 are reused across organizations and across teams within the same organization. That is fine when networks are isolated. The moment you try to merge them, peer them, or bridge them, the duplication becomes a hard blocker.

Scenario One: Merging Two VPCs With Overlapping Ranges

Picture two VPC networks. Network A uses 10.10.0.0/16. Network B uses 10.10.128.0/24. Network B's range falls entirely inside Network A's range, which means some private IPs are potentially in use on both sides. You cannot merge them as-is.

The resolution path looks like this:

1. Stand up a temporary connection between the two networks using Cloud VPN. This lets traffic continue to flow during the migration.
2. Pick a new IP range for one of the networks, usually the one being absorbed. Choose the range with the final merged topology in mind, not just the immediate non-overlap requirement.
3. Migrate all resources in Network B to the new range. Update VM network configuration, DNS records, firewall rules, and any application that hardcodes IPs.
4. Once Network B is fully reconfigured, merge the two networks under a single unified range. In the canonical example, 10.10.0.0/15 is large enough to hold everything from both original networks with room to grow.

The reconfiguration step is the slow part. The Cloud VPN tunnel exists to buy you time during that step, not to be a permanent fix.

Scenario Two: Cloud VPN as an Interim Bridge

This is the same picture from a different angle, and the Professional Cloud Architect exam likes to test the distinction. Cloud VPN does not resolve overlapping IP ranges. It bridges them temporarily so that critical communication can continue while you do the actual remediation.

Three things to internalize about Cloud VPN's role here:

• It is a secure bridging tool, not a routing fix. Traffic flows through encrypted tunnels, but the underlying address conflict is still there.
• It is interim. The long-term answer is always to reconfigure one side's IP ranges so that the conflict goes away entirely.
• It exists to keep applications running during a merger, acquisition, or restructuring while engineering teams do the IP migration work.

If you see an exam question where two networks have overlapping ranges and the question asks how to enable communication between them right now, Cloud VPN is the answer. If the question asks how to permanently integrate the networks, the answer is reconfigure the ranges.

Scenario Three: Cloud Interconnect and On-Prem to GCP

The third place this concept lands is hybrid connectivity through Cloud Interconnect. The principle is the same, but the constraint is tighter because Interconnect is meant to be a long-lived production link, not a temporary bridge.

Imagine an on-premises network using 192.168.0.0/16 and a GCP VPC also using 192.168.0.0/16. When you bring up the Interconnect, the network has no reliable way to distinguish a local on-prem host from a remote cloud host with the same address. Routing breaks.

The fix is straightforward but worth stating plainly: pick distinct, non-overlapping ranges before you build the link. Keep the on-prem network on 192.168.0.0/16 and assign the GCP VPC something like 10.0.0.0/16. With separate ranges, routers can cleanly direct traffic to the correct destination and the Interconnect operates as intended.

This is something to plan for at the design stage. Retrofitting non-overlapping ranges after the fact is the same painful migration described in scenario one.

What to Take Into the Exam

The Professional Cloud Architect exam can test any of the three angles, and the right answer depends on what the question is actually asking:

• Two VPCs need to merge but have overlapping ranges. Use Cloud VPN as an interim connection while you reconfigure one side's IP range, then merge.
• Two networks with overlapping ranges need to communicate today. Cloud VPN bridges them. It does not solve the conflict, but it lets traffic flow.
• An on-prem network is being connected to GCP via Cloud Interconnect. Design with distinct, non-overlapping IP ranges from the start. Overlap will cause routing failures.

The common thread is that overlap is a real problem that prevents networks from being directly integrated, and the long-term fix is always IP reconfiguration. Cloud VPN is the bridge you use while that reconfiguration is in flight.

My Professional Cloud Architect course covers IP range overlap resolution alongside the rest of the networking material.