Private Const WSADescription_Len As Long = 256
Private Const WSASYS_Status_Len As Long = 128
Private Const WS_VERSION_REQD As Long = &H101
Private Const IP_SUCCESS As Long = 0
Private Const SOCKET_ERROR As Long = -1
Private Const AF_INET As Long = 2

Private Type WSADATA
  wVersion As Integer
  wHighVersion As Integer
  szDescription(0 To WSADescription_Len) As Byte
  szSystemStatus(0 To WSASYS_Status_Len) As Byte
  imaxsockets As Integer
  imaxudp As Integer
  lpszvenderinfo As Long
End Type

Private Declare Function WSAStartup Lib "wsock32" (ByVal VersionReq As Long, WSADataReturn As WSADATA) As Long 
Private Declare Function WSACleanup Lib "wsock32" () As Long
Private Declare Function inet_addr Lib "wsock32"  (ByVal s As String) As Long
Private Declare Function gethostbyaddr Lib "wsock32" (haddr As Long, ByVal hnlen As Long, ByVal addrtype As Long) As Long
Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (xDest As Any, xSource As Any, ByVal nbytes As Long)  
Private Declare Function lstrlen Lib "kernel32" Alias "lstrlenA" (lpString As Any) As Long
  
  
Public Function SocketsInitialize() As Boolean
   Dim WSAD As WSADATA   
   SocketsInitialize = WSAStartup(WS_VERSION_REQD, WSAD) = IP_SUCCESS    
End Function


Public Sub SocketsCleanup()   
   If WSACleanup() <> 0 Then
       MsgBox "Windows Sockets error occurred in Cleanup.", vbExclamation
   End If    
End Sub


Public Function GetHostNameFromIP(ByVal sAddress As String) As String
   Dim ptrHosent As Long
   Dim hAddress As Long
   Dim nbytes As Long
   
   If SocketsInitialize() Then

     'convert string address to long
      hAddress = inet_addr(sAddress)
      
      If hAddress <> SOCKET_ERROR Then
         
        'obtain a pointer to the HOSTENT structure
        'that contains the name and address
        'corresponding to the given network address.
         ptrHosent = gethostbyaddr(hAddress, 4, AF_INET)
   
         If ptrHosent <> 0 Then
         
           'convert address and
           'get resolved hostname
            CopyMemory ptrHosent, ByVal ptrHosent, 4
            nbytes = lstrlen(ByVal ptrHosent)
         
            If nbytes > 0 Then
               sAddress = Space$(nbytes)
               CopyMemory ByVal sAddress, ByVal ptrHosent, nbytes
               GetHostNameFromIP = sAddress
            End If
         
         Else: MsgBox "Call to gethostbyaddr failed."
         End If 'If ptrHosent
      
      SocketsCleanup
      
      Else: MsgBox "String passed is an invalid IP."
      End If 'If hAddress
   
   Else: MsgBox "Sockets failed to initialize."
   End If  'If SocketsInitialize      
End Function


Private Sub Command1_Click()
   Text2.Text = GetHostNameFromIP(Text1.Text)   
End Sub

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