Multi Cloud CIDR Planning: Avoid VPC and VNet Overlaps

An overlapping CIDR is easy to create and expensive to remove. A new VPC or VNet can look perfectly healthy inside its own cloud account, yet become unusable when you connect it to a data center, another provider, a shared services network, or a newly acquired business. The failure often appears late, during peering or route exchange, after applications and security rules already depend on the addresses.

The durable solution is to plan private address space as one organization wide system. AWS VPCs, Azure VNets, Google Cloud VPC subnets, branches, VPN gateways, and container ranges all need a place in the same plan. This guide shows how to build that plan, account for provider differences, and put a check in front of every new allocation.

Why CIDR overlap becomes a connectivity problem

CIDR overlap means that two ranges contain at least one identical IP address. For example, 10.20.0.0/16 contains 10.20.8.0/21, so those ranges overlap even though their network addresses look different. A route table cannot reliably decide whether traffic for 10.20.8.25 belongs to the larger network or the smaller one.

Cloud providers enforce this rule at different boundaries. AWS VPC peering does not allow overlapping IPv4 or IPv6 CIDR blocks, and AWS recommends non overlapping private ranges for VPCs connected through Transit Gateway or VPN. Azure does not allow peering between VNets with overlapping address spaces. Google Cloud checks primary and secondary subnet ranges when VPC networks are peered and also considers routes imported from on premises connections.

That means an isolated network can tolerate reuse that a connected network cannot. Treat isolation as a temporary condition, not as permission to reuse the range. The network that is separate today may be the one you need to connect during a migration, incident, acquisition, or disaster recovery test.

Design the global address plan first

Start with an inventory, not a provider console. Collect every range that can appear in a route table or a network policy: data centers, branch offices, remote access pools, existing VPCs and VNets, VPN translation pools, partner connections, management networks, Kubernetes pod and service ranges, and reserved blocks that have not been deployed yet. Include ranges owned by another team if your network may connect to them.

Then choose an organization wide address budget. RFC 1918 defines 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 for private internets, but the fact that each organization can reuse them does not make reuse safe inside your own connected estate. Pick a hierarchy that leaves room for growth, aggregation, and acquisitions. Do not allocate the first unused /24 to a new cloud project just because it is available in one console.

The parent block should have a meaning that survives changes in ownership. A provider block can contain regions, a regional block can contain environments, and an environment block can contain VPCs or VNets. This gives you predictable summaries for routing and makes it easier to answer whether a proposed range belongs to the right part of the plan.

Split each provider block predictably

Use the same allocation pattern in every provider block. For example, a /12 provider block might contain /16 regional blocks, /20 environment networks, and /24 or larger subnets for specific functions. The numbers are examples, not a universal standard. Your sizing should come from workload counts, service requirements, growth forecasts, and the amount of summarization you need.

AWS has VPC CIDR rules that affect peering and transit, so reserve room for multiple VPCs and secondary CIDRs before the first account is created. Azure VNets are regional, and Azure recommends planning non overlapping ranges across primary and backup regions. Google Cloud VPC networks are global, but their subnets are regional, and a subnet can have secondary ranges that are used by alias IPs and GKE.

Keep infrastructure subnets visible in the same plan as application subnets. VPN gateways, firewalls, load balancers, private endpoints, management planes, and shared services all consume address space. Azure also reserves five addresses in each subnet, while Google Cloud reserves a virtual gateway address in the primary range. These details make a theoretical capacity calculation different from the number of addresses your application can actually use.

Treat secondary and overlay ranges as first class allocations

Many overlap incidents happen because the team plans only the VPC or VNet address space. Container platforms then introduce pod and service ranges, platform teams add secondary subnet ranges, and a VPN team reserves a translation pool in a separate document. Those ranges still participate in routing, so they must be checked against the global plan.

For GKE, record every secondary range associated with a subnet, not just the primary range. Google Cloud creates subnet routes for primary and secondary ranges and prevents conflicting ranges in several connected scenarios. For AKS, record the pod and service address plans according to the networking mode you use. The same principle applies to EKS, Kubernetes clusters running on premises, and any overlay network that can be advertised through a router.

Use a naming convention that makes the scope obvious. A record such as gcp-prod-us-central1-gke-pods tells you more than a bare 10.40.0.0/16, especially when an engineer is reviewing a proposed route from a different cloud. Add the owner, environment, region, purpose, parent block, and lifecycle status to every allocation.

Validate every allocation before it ships

The overlap test is simple, but the input set is not. A correct check compares the proposed CIDR with all existing, planned, and reserved CIDRs in the scopes that may connect. It must recognize both exact matches and containment. 10.30.0.0/16 and 10.30.0.0/24 overlap, as do 10.30.0.0/24 and 10.30.0.128/25.

Put the check in three places. First, make the allocation registry authoritative so engineers request space before writing Terraform. Second, run a CI check against the registry and the proposed infrastructure change. Third, enforce provider side controls where they exist, such as AWS IPAM pools or Azure Virtual Network Manager policies. Provider controls are useful guardrails, but they do not replace a global plan because each provider sees only its own address space.

For a quick review of an existing plan, use the subnet overlap checker to find intersecting ranges, then use the IP allocation planner to carve named blocks from a parent network. For a team wide source of truth, an IPAM platform such as Obelinf’s IP address management can keep cloud and on premises allocations in one hierarchy, where a proposed child range is checked against the rest of the organization rather than a single account.

What to do when ranges already overlap

Do not assume that NAT makes an overlap harmless. NAT can be a practical bridge for a constrained migration or a vendor connection, but it adds translation rules, asymmetric routing risks, troubleshooting complexity, and a second address vocabulary that every application owner must understand.

The cleanest fix is usually to renumber the smaller or less populated network before it becomes a dependency. Inventory the workloads, lower DNS and application TTLs, create the replacement range, migrate in stages, and keep the old range during a controlled transition. If renumbering is not possible, isolate the networks and document exactly which translated prefixes are allowed across the boundary.

Before a peering, VPN, Transit Gateway, ExpressRoute, Cloud Interconnect, or Shared VPC change, run a connectivity review that includes routes in both directions. Confirm that the proposed range is not only unused, but also absent from every connected network, reserved block, firewall rule, DNS view, and container overlay plan.

The practical rule for multi cloud CIDR planning

Allocate private address space from the organization outward, not from individual cloud consoles inward. Inventory first, reserve parent blocks, use the same hierarchy in every provider, record secondary and overlay ranges, and reject overlaps before provisioning. That process turns a late connectivity surprise into an early planning decision.

The next useful step is to test your current ranges against every range you may connect in the next two years, not just the networks connected today. A CIDR that is safe in isolation is not necessarily safe in a multi cloud architecture, and the cost of reserving space now is almost always lower than the cost of renumbering a live environment later.