How can you be so sure that you only have one NIC with an IP Address bound to it? You can loop through all the network adapters.
;*************************************************************************
;  Script Name:   LoopNICs
;  Author:        Wim Rotty
;  Date:          11/12/2007
;  Description:   Loop all network adapters with IP Address
;*************************************************************************
 
;Script Options
If Not @LOGONMODE
    Break On
Else
    Break Off
EndIf
Dim $RC
$RC = SetOption("Explicit", "On")
$RC = SetOption("NoMacrosInStrings", "On")
$RC = SetOption("NoVarsInStrings", "On")
If @SCRIPTEXE = "KIX32.EXE"
    $RC = SetOption("WrapAtEOL", "On")
EndIf

;Declare variables
Dim $i, $IPAddress

;Initialize variables
$i = 0

;Code
$IPAddress = EnumIPInfo($i,0,1)
While Not @ERROR
    If Not $IPAddress = "0.0.0.0"
        "IP Adderss: " $IPAddress ?
        "Subnet Mask: " EnumIPInfo($i,1,1) ?
        "Adapter Description: " EnumIPInfo($i,2,1) ?
        "Default Gateway: " EnumIPInfo($i,3,1) ?
        ?
    EndIf
    $i = $i + 1
    $IPAddress = EnumIPInfo($i,0,1)
Loop
;Personal UDF Section

;UDF Section