Lab: 39 Subnet Migration

What is Subnet Migration

** Subnet migration is the process of moving resources from one network subnet to another to improve security, organization, or network design.**

Subbnet Migration is an OCI feature that allows you to convert a subnet between two scope types — regional and AD-specific (Availability Domain-specific) — without deleting and recreating it. Historically, early OCI tenancies had only AD-specific subnets. As Oracle evolved its platform, regional subnets became the recommended standard because they span all ADs within a region, giving better flexibility and resilience. Subnet Migration lets you upgrade your existing AD-specific subnets to regional scope in-place. Why it matters: Regional subnets allow instances to be placed in any AD within the region using a single subnet, simplifying architecture and enabling true HA designs without managing multiple subnet CIDRs.

Simple Example

Suppose a VM is currently in:

Subnet A (Development Network)

and you want it to be in:

Subnet B (Production Network)

Moving the VM or recreating it in Subnet B is called subnet migration.

Key benefits:

• No CIDR change, no IP reassignment — resources keep their IPs
• No downtime required during migration
• Enables HA across all ADs from one subnet
• Simplifies network design — fewer subnets to manage
• Free, native OCI operation

Key drawbacks / caveats:

• One-way only — you cannot convert a regional subnet back to AD-specific
• All resources in the subnet must be compatible before migrating
• Some legacy resource types may block migration
• Requires careful validation — OCI checks compatibility before allowing the change
• Does not change CIDR, security lists, or route tables — those carry over as-is

Here is the before-and-after structural diagram showing exactly what changes during a subnet migration: