Azure Networking – Subnets

What is a Subnet?

A subnet is a range of IP addresses in your virtual network. Subnets enable you to segment your virtual network into smaller, manageable sections, apply network security policies, and allocate resources efficiently.

Key Concepts

• Address space: The CIDR block assigned to a virtual network (e.g., 10.0.0.0/16).
• Subnet CIDR: A subset of the virtual network’s address space (e.g., 10.0.1.0/24).
• Network Security Groups (NSGs): Bindable to subnets for traffic filtering.
• Service Endpoints: Extend Azure services into a subnet.

Creating a Subnet (Azure CLI)

az network vnet subnet create \
  --resource-group MyResourceGroup \
  --vnet-name MyVNet \
  --name MySubnet \
  --address-prefix 10.0.1.0/24 \
  --network-security-group MyNSG \
  --service-endpoints Microsoft.Storage

Creating a Subnet (PowerShell)

$subnet = New-AzVirtualNetworkSubnetConfig `
    -Name "MySubnet" `
    -AddressPrefix "10.0.1.0/24" `
    -NetworkSecurityGroup $nsg `
    -ServiceEndpoint @("Microsoft.Storage")
New-AzVirtualNetwork -Name "MyVNet" -ResourceGroupName "MyResourceGroup" `
    -Location "EastUS" -AddressPrefix "10.0.0.0/16" -Subnet $subnet

ARM Template Example

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "resources": [
    {
      "type": "Microsoft.Network/virtualNetworks",
      "apiVersion": "2022-07-01",
      "name": "myVNet",
      "location": "[resourceGroup().location]",
      "properties": {
        "addressSpace": { "addressPrefixes": [ "10.0.0.0/16" ] },
        "subnets": [
          {
            "name": "mySubnet",
            "properties": {
              "addressPrefix": "10.0.1.0/24",
              "networkSecurityGroup": {
                "id": "[resourceId('Microsoft.Network/networkSecurityGroups', 'myNSG')]"
              },
              "serviceEndpoints": [
                { "service": "Microsoft.Storage" }
              ]
            }
          }
        ]
      }
    }
  ]
}

Subnet Limits

Best Practices

• Plan address space to avoid overlap with on-premises networks.
• Reserve address ranges for future use.
• Assign NSGs per subnet for logical security zones.
• Use service endpoints to secure Azure service access.
• Keep subnets small enough for manageable IP allocation.