findif.ps1

# invoke as:
#  powershell -NoProfile -ExecutionPolicy Unrestricted -NonInteractive .\findif.ps1
#

Param(
 [Parameter(Mandatory=$True,Position=1)][string]$IPAddress, 
 [Parameter(Mandatory=$True,Position=2)][string]$InterfaceAddress
)

# check if the ipaddr is already assigned
$ifindex = (Get-NetIPAddress | where IPAddress -eq $IPAddress).InterfaceIndex
Write-Host "ifindex = " $ifindex

if ($ifindex -eq $null) {
  # find the interface by InterfaceAddress
  $ifindex = (Get-NetIPAddress | where IPAddress -eq $InterfaceAddress).InterfaceIndex
  Write-Host "ifindex = " $ifindex
}