Enumerating IPs

This article is based on Visual Basic 6. Find other Visual Basic 6 articles.

If you need to know all the IP addresses allocated to the local machine, there is a special procedure to go through as follows:

1. Get the local host name
2. Get the host information for the local host name
3. Read all the listed IPs

Unfortunately, this involves the Hostent structure which is very hard to deal with in VB.

Various API calls and structures are needed for this function:

Private Type WSAData
  wVersion As Integer
  wHighVersion As Integer
  szDescription(0 To 256) As Byte 'WSADESCRIPTION_LEN
  szSystemStatus(0 To 128) As Byte 'WSASYS_STATUS_LEN
  wMaxSockets As Integer
  wMaxUDPDG As Integer
  dwVendorInfo As Long
End Type

Private Type HOSTENT
  hName As Long
  hAliases As Long
  hAddrType As Integer
  hLen As Integer
  hAddrList As Long
End Type

Private Declare Function WSAStartup Lib "WSOCK32" (ByVal wVersionRequired As Long, _
  lpWSADATA As WSAData) As Long
Private Declare Function WSACleanup Lib "WSOCK32" () As Long
Private Declare Function gethostname Lib "WSOCK32" (ByVal szHost As String, _
  ByVal dwHostLen As Long) As Long
Private Declare Function gethostbyname Lib "WSOCK32" (ByVal szHost As String) As Long
Private Declare Function inet_ntoa Lib "ws2_32.dll" (ByVal laddr As Long) As Long

Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (pDst As Any, pSrc As Any, _
  ByVal ByteLen As Long)

Private Declare Function lstrlen Lib "kernel32" Alias "lstrlenA" (ByVal lpString As Long) As Long
Private Declare Function lstrcpy Lib "kernel32" Alias "lstrcpyA" (ByVal lpString1 As String, _
  lpString2 As Any) As Long

And now the function itself:

Public Function EnumurateIPs(IPs() As String) As Long
Dim WSData As WSAData
Dim HostName As String
Dim HostInfoPointer As Long
Dim HostInfo As HOSTENT
Dim IPPointer As Long
Dim IPLong As Long
Dim IPAddressP As Long
Dim IPAddress As String
Dim Count As Long
    
  'Initialise everything
  Count = 0
  IPs = Split("", "")
  If WSAStartup(&H101, WSData) <> 0 Then Exit Function
  
  'Get the local host name
  HostName = String(256, vbNullChar)
  If gethostname(HostName, Len(HostName)) <> -1 Then
    If InStr(HostName, vbNullChar) > 0 Then HostName = Left(HostName, InStr(HostName, vbNullChar) - 1)
    
    'Lookup the information for the local host
    HostInfoPointer = gethostbyname(HostName)
    If HostInfoPointer <> 0 Then
      'Copy it into a nice hostent structure
      CopyMemory HostInfo, ByVal HostInfoPointer, Len(HostInfo)
      Do
        'Copy the IP address pointer out of the "array"
        CopyMemory IPPointer, ByVal HostInfo.hAddrList + (4 * Count), 4
        'Loop until we find a null item
        If IPPointer = 0 Then Exit Do
        
        'Copy the IP itself into a variable
        CopyMemory IPLong, ByVal IPPointer, HostInfo.hLen
        'Convert the pointer to an IP string
        IPAddressP = inet_ntoa(IPLong)
        IPAddress = String(lstrlen(ByVal IPAddressP), vbNullChar)
        Call lstrcpy(IPAddress, ByVal IPAddressP)
        
        'Add it to the list
        ReDim Preserve IPs(0 To Count)
        IPs(Count) = IPAddress
        Count = Count + 1
      Loop
    End If
  End If
  
  'Clean up
  WSACleanup
  EnumurateIPs = Count
End Function