devops-network-calculator-for-azure

---
name: devops-network-calculator-for-azure
description: Azure network planning, CIDR calculation, subnet sizing, and best-practices tool. Use this skill whenever the user asks about subnet sizing, CIDR planning, AKS networking, NSG rules, network segmentation, IP address management, VNet planning, address space analysis, overlap detection, or any Azure networking topic. Also trigger when the user mentions network calculator, net-calc, calculate hosts, plan subnets, or asks about Azure network best practices, even if they don't explicitly say 'network calculator'.
license: MIT
---

# Azure Network Calculator

Offline Azure network planning tool. Calculates CIDRs, detects overlaps, analyzes VNet utilization, plans AKS networking, and generates Terraform-ready output. Zero external dependencies — uses Python stdlib only.

## Quick Start

```bash
# What CIDR do I need for 500 hosts?
python3 scripts/network-calc.py calculate --from-hosts 500

# Analyze current VNet
python3 scripts/network-calc.py analyze --from-tfvars terraform/terraform.tfvars

# Validate for overlaps (pre-commit compatible)
python3 scripts/network-calc.py validate --from-tfvars terraform/terraform.tfvars

# Find where to place a new subnet
python3 scripts/network-calc.py first-fit --vnet 10.248.0.0/20 \
  --subnets "10.248.0.0/22,10.248.4.0/22,10.248.8.0/26,10.248.9.0/24" --hosts 500
```

## Commands

| Command | Purpose | Reference |
|---------|---------|-----------|
| `calculate` | CIDR info, host sizing, subnet splitting | [CIDR Guide](./references/cidr-calculation-guide.md) |
| `analyze` | VNet utilization, gap analysis | [CIDR Guide](./references/cidr-calculation-guide.md) |
| `validate` | Overlap detection, Azure constraint checks | [Azure Constraints](./references/azure-constraints.md) |
| `first-fit` | Find optimal placement for new subnet | [CIDR Guide](./references/cidr-calculation-guide.md) |
| `plan-multi` | Multi-environment VNet allocation | [Segmentation](./references/segmentation-patterns.md) |

## Project Context

This project's current VNet: `10.248.0.0/20` (4,096 IPs, 57.8% utilized)

| Subnet | CIDR | Usable |
|--------|------|--------|
| GatewaySubnet | 10.248.0.0/22 | 1,019 |
| PublicSubnet | 10.248.4.0/22 | 1,019 |
| AzureBastionSubnet | 10.248.8.0/26 | 59 |
| PrivateSubnet | 10.248.9.0/24 | 251 |
| **Available gaps** | | **1,708** |

Key files: `terraform/terraform.tfvars`, `terraform/networking.tf`, `terraform/nsg.tf`

## Azure Quick Reference

- **5 reserved IPs** per subnet (.0, .1, .2, .3, broadcast)
- **Bastion:** min /26 | **Gateway:** min /27 | **Firewall:** min /26
- **Max subnets/VNet:** 3,000 | **Max NSG rules:** 1,000
- Full reference: [Azure Constraints](./references/azure-constraints.md)

## Reference Guides

| Guide | When to Read |
|-------|-------------|
| [CIDR Calculation Guide](./references/cidr-calculation-guide.md) | Subnet sizing, gap analysis, overlap detection |
| [AKS Networking Guide](./references/aks-networking-guide.md) | CNI comparison, pod/service CIDR, node sizing |
| [Segmentation Patterns](./references/segmentation-patterns.md) | Design patterns, anti-patterns, decision matrices |
| [Azure Constraints](./references/azure-constraints.md) | Hard limits, naming rules, reserved addresses |

## Templates

| Template | Purpose |
|----------|---------|
| [VNet Layout](./templates/vnet-layout.tfvars.tpl) | Terraform variable blocks for VNet config |
| [AKS NSG Rules](./templates/nsg-rules-aks.tfvars.tpl) | NSG rules for AKS workloads |
| [Multi-Env Plan](./templates/multi-env-plan.md.tpl) | Multi-environment planning output |

## Execution

Follow the instructions in `./workflow.md`.

---

## Gotchas

- **Azure reserves 5 IPs per subnet, not 2:** A standard /29 gives you 3 usable IPs, not 6. Sizing AKS node pools with /29 starves scaling and surfaces as `InsufficientFreeAddressesInSubnet` mid-scale.
- **GatewaySubnet name is literal:** Azure refuses VPN Gateway attach unless the subnet is named exactly `GatewaySubnet`. Same for `AzureBastionSubnet` and `AzureFirewallSubnet`. Typo breaks at apply, not plan.
- **AKS Azure CNI eats one subnet IP per pod:** A 30-node cluster at 30 pods/node needs 900+ subnet IPs, not 30. Plan `--max-pods` first; switching CNI later requires cluster rebuild.
- **first-fit picks lowest gap, not smartest:** Places a /24 in a /22 gap and leaves fragmented unusable /25 leftovers. For long-lived VNets, use `plan-multi` and reserve growth gaps explicitly.
- **CIDR overlap with on-prem fails silently until peering:** A 10.0.0.0/16 VNet looks fine alone, but ExpressRoute peering black-holes routes overlapping corporate ranges. Validate against full corporate IPAM.
- **NSG rule limit is 1,000 total, not per-direction:** Inbound + outbound + default + Azure-auto rules share the budget. Leave headroom or hit a hard wall during scale.