Virtual Network Subnet name must be valid

April 25, 2025 · View on GitHub

SYNOPSIS

Azure Resource Manager (ARM) has requirements for Virtual Network Subnet names.

DESCRIPTION

When naming Azure resources, resource names must meet service requirements. The requirements for Route table names are:

Between 1 and 80 characters long.
Alphanumerics, underscores, periods, and hyphens.
Start with alphanumeric.
End alphanumeric or underscore.
Subnet names must be unique within a virtual network.

RECOMMENDATION

Consider using names that meet subnet naming requirements. Additionally consider naming resources with a standard naming convention.

EXAMPLES

Configure with Bicep

To deploy virtual network subnets that pass this rule:

Set the name property to a string that matches the naming requirements.
Optionally, consider constraining name parameters with minLength and maxLength attributes.

For example:

@minLength(1)
@maxLength(80)
@description('The name of the resource.')
param name string

resource subnet 'Microsoft.Network/virtualNetworks/subnets@2024-05-01' = {
  parent: vnet
  name: name
  properties: {
    addressPrefix: '10.0.0.0/24'
    networkSecurityGroup: {
      id: nsg.id
    }
    defaultOutboundAccess: false
  }
}

Configure with Azure template

To deploy virtual network subnets that pass this rule:

Set the name property to a string that matches the naming requirements.
Optionally, consider constraining name parameters with minLength and maxLength attributes.

For example:

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "metadata": {
    "_generator": {
      "name": "bicep",
      "version": "0.34.44.8038",
      "templateHash": "1920943566989111543"
    }
  },
  "parameters": {
    "name": {
      "type": "string",
      "minLength": 1,
      "maxLength": 80,
      "metadata": {
        "description": "The name of the resource."
      }
    }
  },
  "resources": [
    {
      "type": "Microsoft.Network/virtualNetworks/subnets",
      "apiVersion": "2024-05-01",
      "name": "[format('{0}/{1}', parameters('name'), parameters('name'))]",
      "properties": {
        "addressPrefix": "10.0.0.0/24",
        "networkSecurityGroup": {
          "id": "[resourceId('Microsoft.Network/networkSecurityGroups', parameters('name'))]"
        },
        "defaultOutboundAccess": false
      },
      "dependsOn": [
        "[resourceId('Microsoft.Network/networkSecurityGroups', parameters('name'))]",
        "[resourceId('Microsoft.Network/virtualNetworks', parameters('name'))]"
      ]
    }
  ]
}

NOTES

This rule does not check if subnet names are unique.