Subnet creation

 $resourceGroup = "exam-grp"

$network = "exam-network"

$subnetName = "SubnetA"

$subnetAddressPrefix="10.1.0.0/24"

$virtualNetwork=get-azvirtualnetwork -Name $network -ResourceGroupName $resourceGroup 

Add-azvirtualnetworksubnetconfig -Name $subnetName -VirtualNetwork $virtualNetwork -AddressPrefix $subnetAddressPrefix

$virtualNetwork  |  Set-AzVirtualNetwork

#or can create subnet during vnet creation

$subnet= New-azvirtualnetworksubnetconfig -Name $subnetName -AddressPrefix $subnetAddressPrefix

New-AzVirtualNetwork -Name $network -ResourceGroupName $resourceGroup -Location $location -AddressPrefix $AddressPrefix -Subnet $subnet