Get host name from IP address, and vice versa

A common question is 'How do I resolve a host name into an IP address', or 'How to I get an IP address' host name'? Fortunately, this is possible without using the WinSock control, or anything else... just plain Windows API. To make the process as simple as possible, we've put all the code into a class module which you can then easily use in your application. This class module has a GetIPFromHostName and a GetHostNameFromIP function. Below is the code for the class module. Call it clsResolve.

Option Explicit

'// define constants
Private Const IP_SUCCESS As Long = 0
Private Const SOCKET_ERROR As Long = -1

Private Const MAX_WSADescription As Long = 256
Private Const MAX_WSASYSStatus As Long = 128
Private Const MIN_SOCKETS_REQD As Long = 1

Private Const WS_VERSION_REQD As Long = &H101
Private Const WS_VERSION_MAJOR As Long = WS_VERSION_REQD \ &H100 And &HFF&

Private Const WSADescription_Len As Long = 256
Private Const WSASYS_Status_Len As Long = 128
Private Const AF_INET As Long = 2

'// structures

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

Private Type WSADATA
   wVersion As Integer
   wHighVersion As Integer
   szDescription(0 To MAX_WSADescription) As Byte
   szSystemStatus(0 To MAX_WSASYSStatus) As Byte
   wMaxSockets As Long
   wMaxUDPDG As Long
   dwVendorInfo As Long
End Type

'// api
Private Declare Sub apiCopyMemory Lib "kernel32" Alias "RtlMoveMemory" (xDest As Any, xSource As Any, ByVal nBytes As Long)
Private Declare Function apiStrLen Lib "kernel32" Alias "lstrlenA" (lpString As Any) As Long
Private Declare Function apiGetHostByName Lib "wsock32.dll" Alias "gethostbyname" (ByVal hostname As String) As Long
Private Declare Function apiWSAStartup Lib "wsock32.dll" Alias "WSAStartup" (ByVal wVersionRequired As Long, lpWSADATA As WSADATA) As Long
Private Declare Function apiWSACleanup Lib "wsock32.dll" Alias "WSACleanup" () As Long
Private Declare Function apiInetAddr Lib "wsock32.dll" Alias "inet_addr" (ByVal s As String) As Long
Private Declare Function apiGetHostByAddr Lib "wsock32.dll" Alias "gethostbyaddr" (haddr As Long, ByVal hnlen As Long, ByVal addrtype As Long) As Long
'// private functions
Private Function InitializeSocket() As Boolean
    'attempt to initialize the socket
    InitializeSocket = apiWSAStartup(WS_VERSION_REQD, WSAD) = IP_SUCCESS
End Function

Private Sub CloseSocket()
    'try to close the socket
    If apiWSACleanup() <> 0 Then
        MsgBox "Error calling apiWSACleanup.", vbCritical
    End If

End Sub

Public Function GetIPFromHostName(ByVal sHostName As String) As String
    'converts a host name to an IP address.
    Dim nBytes As Long
    Dim ptrHosent As Long
    Dim hstHost As HOSTENT
    Dim ptrName As Long
    Dim ptrAddress As Long
    Dim ptrIPAddress As Long
    Dim sAddress As String 'declare this as Dim sAddress(1) As String if you want 2 ip addresses returned
    'try to initalize the socket
    If InitializeSocket() = True Then
        'try to get the IP
        ptrHosent = apiGetHostByName(sHostName & vbNullChar)
        If ptrHosent <> 0 Then
            'get the IP address
            apiCopyMemory hstHost, ByVal ptrHosent, LenB(hstHost)
            apiCopyMemory ptrIPAddress, ByVal hstHost.hAddrList, 4
            'fill buffer
            sAddress = Space$(4)
            'if you want multiple domains returned,
            'fill all items in sAddress array with 4 spaces
            apiCopyMemory ByVal sAddress, ByVal ptrIPAddress, hstHost.hLength
            'change this to
            'CopyMemory ByVal sAddress(0), ByVal ptrIPAddress, hstHost.hLength
            'if you want an array of ip addresses returned
            '(some domains have more than one ip address associated with it)
            'get the IP address
            GetIPFromHostName = IPToText(sAddress)
            'if you are using multiple addresses, you need IPToText(sAddress(0)) & "," & IPToText(sAddress(1))
        End If
        MsgBox "Failed to open Socket."
    End If
End Function

Private Function IPToText(ByVal IPAddress As String) As String
    'converts characters to numbers
    IPToText = CStr(Asc(IPAddress)) & "." & _
              CStr(Asc(Mid$(IPAddress, 2, 1))) & "." & _
              CStr(Asc(Mid$(IPAddress, 3, 1))) & "." & _
              CStr(Asc(Mid$(IPAddress, 4, 1)))
End Function

Public Function GetHostNameFromIP(ByVal sIPAddress As String) As String
    Dim ptrHosent As Long
    Dim hAddress As Long
    Dim sHost As String
    Dim nBytes As Long
    'try to open the socket
    If InitializeSocket() = True Then
        'convert string address to long datatype
        hAddress = apiInetAddr(sIPAddress)
        'check if an error ocucred
        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 = apiGetHostByAddr(hAddress, 4, AF_INET)
            If ptrHosent <> 0 Then
                'convert address and
                'get resolved hostname

                apiCopyMemory ptrHosent, ByVal ptrHosent, 4
                nBytes = apiStrLen(ByVal ptrHosent)
                If nBytes > 0 Then
                    'fill the IP address buffer
                    sHost = Space$(nBytes)
                    apiCopyMemory ByVal sHost, ByVal ptrHosent, nBytes
                    GetHostNameFromIP = sHost
                End If
                MsgBox "Call to gethostbyaddr failed."
            End If
            'close the socket
            MsgBox "Invalid IP address"
        End If
        MsgBox "Failed to open Socket"
    End If
End Function

Using the class module is very simple. The following code gets the IP address of, and resolves

Private Sub cmdResolve_Click()
Dim cResolve As clsResolve
    Set cResolve = New clsResolve
    MsgBox cResolve.GetHostNameFromIP("")
    MsgBox cResolve.GetIPFromHostName("")
End Sub

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James Crowley James first started this website when learning Visual Basic back in 1999 whilst studying his GCSEs. The site grew steadily over the years while being run as a hobby - to a regular monthly audience ...


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