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Introduction / Theory:

"Azure Virtual Network is the fundamental building block for your private network in Azure. A virtual network enables many types of Azure resources, such as Azure Virtual Machines (VM), to securely communicate with each other, the internet, and on-premises networks. A virtual network is similar to a traditional network that you'd operate in your own data center. An Azure Virtual Network brings with it extra benefits of Azure's infrastructure such as scale, availability, and isolation."

Microsoft

Example: Azure virtual network

Create Virtual Network via Portal:


Create Virtual Network via PowerShell:

# Ressource Group Creation
$rg = @{
    Name = 'rg-avd-test'
    Location = 'WestEurope'
}
New-AzResourceGroup @rg
# VNet Creation
$vnet = @{
    Name = 'avdtestvnet01'
    ResourceGroupName = 'rg-avd-test'
    Location = 'WestEurope'
    AddressPrefix = '10.0.0.0/16'    
}
$virtualNetwork = New-AzVirtualNetwork @vnet
# Subnet Creation
$subnet = @{
    Name = 'default'
    VirtualNetwork = $virtualNetwork
    AddressPrefix = '10.0.0.0/24'
}
$subnetConfig = Add-AzVirtualNetworkSubnetConfig @subnet

# Write the subnet configuration to the virtual network 
$virtualNetwork | Set-AzVirtualNetwork

Via Azure CLI:

# Ressource Group Creation
az group create \
    --name rg-avd-test \
    --location westeurope
# VNet- and  Subnet - Creation
az network vnet create \
  --name avdtestvnet01 \
  --resource-group rg-avd-test \
  --subnet-name default

References:

Quickstart: Create a virtual network - Azure portal - Azure Virtual Network
In this quickstart, learn how to create a virtual network using the Azure portal.
Create a virtual network - quickstart - Azure PowerShell - Azure Virtual Network
In this quickstart, you create a virtual network using the Azure portal. A virtual network lets Azure resources communicate with each other and with the internet.
Create a virtual network - quickstart - Azure CLI - Azure Virtual Network
In this quickstart, learn to create a virtual network using the Azure CLI. A virtual network lets Azure resources communicate with each other and with the internet.