Why do I need address prefixes separately from subnets?

IBM Cloud VPC requires you to declare an address prefix per zone — a CIDR block reserved for that zone's subnets. You then carve subnets out of the prefix. The two-level structure prevents subnets in different zones from being allocated from the same range, which would break zone-level routing. Most engineers expect a flat structure and trip over this on day one.

What is the minimum subnet size?

IBM Cloud VPC enforces a /28 minimum (16 addresses, of which IBM reserves 5 — leaving 11 usable). Going smaller than /28 is rejected by the API. For load balancer subnets, /27 is typically the minimum sensible size since LBs may need to scale into more addresses.

IBM VPC Subnet Planner

NetworkingIBM Cloud

Plan VPC subnet layouts with address prefixes, zones, public gateways, and security groups.

Last verified: May 2026

VPC Subnet Configuration

VPC CIDR Block

Number of Subnets

Subnet Prefix Length

IBM VPC reserves 5 IPs per subnet: network address, default gateway, DNS resolver, future use, and broadcast.

Raw Output

Output will appear here...

See It in Action

Your team is designing the production VPC for a new region. The existing primary VPC uses 10.20.0.0/16 across three zones. You enter the new VPC's intended layout into the planner: 10.40.0.0/16, with each zone getting a /18 prefix. The planner flags that your proposed shared-services subnet (10.40.250.0/24) is outside the declared per-zone prefixes — a copy-paste error from the primary VPC's layout. Fix the prefix, re-validate, apply.

What This Tool Does

IBM Cloud VPC requires planning subnets across zones, with address prefixes per zone and per-subnet CIDR carve-outs that must not overlap. The IBM VPC Subnet Planner generates a complete VPC topology — address prefixes, zone-specific subnets, public gateways, and security group bindings — and validates address space against IBM's published limits. Output is Terraform-ready and includes a routing summary so cross-zone communication is explicit.

Technical Details

The planner accepts VPC name, region, and per-zone address prefixes. Within each prefix, you define subnets with names, CIDRs, and optional public gateway attachment. The planner validates each subnet against IBM Cloud's /28 minimum, checks for overlap within and across zones, and generates Terraform `ibm_is_vpc`, `ibm_is_vpc_address_prefix`, and `ibm_is_subnet` resources with explicit dependencies.

Common Use Cases

1Designing a multi-zone VPC for a new production environment with subnets sized for current workloads and future expansion.
2Documenting an existing VPC's IP plan and topology before adding new zones or services.
3Validating that a proposed subnet layout matches your organization's IP allocation standards before applying it.
4Planning IP space for a hybrid setup where VPC subnets must not overlap with on-premise networks reached via Direct Link.

Common Questions

Why do I need address prefixes separately from subnets?: IBM Cloud VPC requires you to declare an address prefix per zone — a CIDR block reserved for that zone's subnets. You then carve subnets out of the prefix. The two-level structure prevents subnets in different zones from being allocated from the same range, which would break zone-level routing. Most engineers expect a flat structure and trip over this on day one.
What is the minimum subnet size?: IBM Cloud VPC enforces a /28 minimum (16 addresses, of which IBM reserves 5 — leaving 11 usable). Going smaller than /28 is rejected by the API. For load balancer subnets, /27 is typically the minimum sensible size since LBs may need to scale into more addresses.

Expert Tips

TIP

Allocate per-zone address prefixes that are larger than your current subnet plan needs. Adding a new prefix later is possible but disruptive; over-provisioning costs nothing because address prefixes themselves do not bill.

TIP

Reserve at least one /24 in each zone for future use. Cross-zone migrations, new tiers, and unplanned services always appear, and a VPC that runs out of IPs forces a much messier rebuild than initial over-allocation.

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