my code
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Public Function AuthenticateUser(ByVal ldapServerName As String, ByVal username As String, ByVal password As String) As Boolean
    Dim ou_ As String = "sunbs.in"
    Dim _ldapServerName = ldapServerName

    Dim oRoot As DirectoryEntry = New DirectoryEntry("LDAP://" & ldapServerName &
"/uid=""" & username & """,ou= ou_", "uid=""" & username & """,ou= ou_", password,   AuthenticationTypes.ServerBind)

    Dim oSearcher As DirectorySearcher = New DirectorySearcher(oRoot)

    Try

        Dim mySearchResult As SearchResult = oSearcher.FindOne()

        Return True


    Catch e As Exception

        MsgBox("Error is " & e.Message)

        Return False

    End Try

End Function