ขอสอบถาม เรื่อง Get All Ip Address On Network in Vb.net

using System;
using System.Runtime.InteropServices;
using System.Security;
using System.Collections;
using System.Windows.Forms;

namespace ListNetworkComputers
{
    #region NetworkBrowser CLASS
    /// <summary>
    /// Provides a mechanism for supplying a list of all PC names in the local network.
    /// This collection of PC names is used in the <see cref="ListNetworkComputers.frmMain">
    /// form </see>
    /// <br></br>
    /// This class makes use of a DllImport instruction. The purpose of which is as follows:
    /// When a DllImport declaration is made in managed code (C#) it is a call to a legacy
    /// unmanaged code module, normally a C++ Dynamic Link Library. These C++ Dll's are
    /// usually part of the operating system API, or some other vendors API, and must be 
    /// used to carry out operations that are not native within the managed code C# framework. 
    /// This is fairly normal within the windows world. The only thing that needs careful consideration
    /// is the construction of the correct type of STRUCTS, object pointers, and attribute markers,
    /// which all contribute to making the link between managed (C#) and unmanaged code (C++)
    /// more seamless
    /// <br></br>
    /// This class makes use of the following Dll calls
    /// <list type="bullet">
    /// <item>
    /// <description> Netapi32.dll : NetServerEnum, The NetServerEnum function lists all servers
    /// of the specified type that are visible in a domain. For example, an application can call 
    /// NetServerEnum to list all domain controllers only or all SQL servers only.
    ///	You can combine bit masks to list several types. For example, a value of 0x00000003 
    ///	combines the bit masks for SV_TYPE_WORKSTATION (0x00000001) and SV_TYPE_SERVER (0x00000002).
    /// </description>
    /// </item>
    /// <item>
    /// <description> Netapi32.dll : NetApiBufferFree, The NetApiBufferFree function frees 
    /// the memory that the NetApiBufferAllocate function allocates. Call NetApiBufferFree 
    /// to free the memory that other network management functions return.</description>
    /// </item>
    /// </list>
    /// </summary>
    public sealed class NetworkBrowser
    {
        #region Dll Imports

        //declare the Netapi32 : NetServerEnum method import
        [DllImport("Netapi32", CharSet = CharSet.Auto, SetLastError = true),
        SuppressUnmanagedCodeSecurityAttribute]

        /// <summary>
        /// Netapi32.dll : The NetServerEnum function lists all servers
        /// of the specified type that are visible in a domain. For example, an 
        /// application can call NetServerEnum to list all domain controllers only
        /// or all SQL servers only.
        ///	You can combine bit masks to list several types. For example, a value 
        /// of 0x00000003  combines the bit masks for SV_TYPE_WORKSTATION 
        /// (0x00000001) and SV_TYPE_SERVER (0x00000002)
        /// </summary>
        public static extern int NetServerEnum(
            string ServerNane, // must be null
            int dwLevel,
            ref IntPtr pBuf,
            int dwPrefMaxLen,
            out int dwEntriesRead,
            out int dwTotalEntries,
            int dwServerType,
            string domain, // null for login domain
            out int dwResumeHandle
            );

        //declare the Netapi32 : NetApiBufferFree method import
        [DllImport("Netapi32", SetLastError = true),
        SuppressUnmanagedCodeSecurityAttribute]

        /// <summary>
        /// Netapi32.dll : The NetApiBufferFree function frees 
        /// the memory that the NetApiBufferAllocate function allocates. 
        /// Call NetApiBufferFree to free the memory that other network 
        /// management functions return.
        /// </summary>
        public static extern int NetApiBufferFree(
            IntPtr pBuf);

        //create a _SERVER_INFO_100 STRUCTURE
        [StructLayout(LayoutKind.Sequential)]
        public struct _SERVER_INFO_100
        {
            internal int sv100_platform_id;
            [MarshalAs(UnmanagedType.LPWStr)]
            internal string sv100_name;
        }
        #endregion
        #region Public Constructor
        /// <summary>
        /// Constructor, simply creates a new NetworkBrowser object
        /// </summary>
        public NetworkBrowser()
        {

        }
        #endregion
        #region Public Methods
        /// <summary>
        /// Uses the DllImport : NetServerEnum with all its required parameters
        /// (see http://msdn.microsoft.com/library/default.asp?url=/library/en-us/netmgmt/netmgmt/netserverenum.asp
        /// for full details or method signature) to retrieve a list of domain SV_TYPE_WORKSTATION
        /// and SV_TYPE_SERVER PC's
        /// </summary>
        /// <returns>Arraylist that represents all the SV_TYPE_WORKSTATION and SV_TYPE_SERVER
        /// PC's in the Domain</returns>
        public ArrayList getNetworkComputers()
        {
            //local fields
            ArrayList networkComputers = new ArrayList();
            const int MAX_PREFERRED_LENGTH = -1;
            int SV_TYPE_WORKSTATION = 1;
            int SV_TYPE_SERVER = 2;
            IntPtr buffer = IntPtr.Zero;
            IntPtr tmpBuffer = IntPtr.Zero;
            int entriesRead = 0;
            int totalEntries = 0;
            int resHandle = 0;
            int sizeofINFO = Marshal.SizeOf(typeof(_SERVER_INFO_100));


            try
            {
                //call the DllImport : NetServerEnum with all its required parameters
                //see http://msdn.microsoft.com/library/default.asp?url=/library/en-us/netmgmt/netmgmt/netserverenum.asp
                //for full details of method signature
                int ret = NetServerEnum(null, 100, ref buffer, MAX_PREFERRED_LENGTH,
                    out entriesRead,
                    out totalEntries, SV_TYPE_WORKSTATION | SV_TYPE_SERVER, null, out 
					resHandle);
                //if the returned with a NERR_Success (C++ term), =0 for C#
                if (ret == 0)
                {
                    //loop through all SV_TYPE_WORKSTATION and SV_TYPE_SERVER PC's
                    for (int i = 0; i < totalEntries; i++)
                    {
                        //get pointer to, Pointer to the buffer that received the data from
                        //the call to NetServerEnum. Must ensure to use correct size of 
                        //STRUCTURE to ensure correct location in memory is pointed to
                        tmpBuffer = new IntPtr((int)buffer + (i * sizeofINFO));
                        //Have now got a pointer to the list of SV_TYPE_WORKSTATION and 
                        //SV_TYPE_SERVER PC's, which is unmanaged memory
                        //Needs to Marshal data from an unmanaged block of memory to a 
                        //managed object, again using STRUCTURE to ensure the correct data
                        //is marshalled 
                        _SERVER_INFO_100 svrInfo = (_SERVER_INFO_100)
                            Marshal.PtrToStructure(tmpBuffer, typeof(_SERVER_INFO_100));

                        //add the PC names to the ArrayList
                        networkComputers.Add(svrInfo.sv100_name);
                    }
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show("Problem with acessing network computers in NetworkBrowser " +
                    "\r\n\r\n\r\n" + ex.Message,
                    "Error", MessageBoxButtons.OK,
                    MessageBoxIcon.Error);
                return null;
            }
            finally
            {
                //The NetApiBufferFree function frees 
                //the memory that the NetApiBufferAllocate function allocates
                NetApiBufferFree(buffer);
            }
            //return entries found
            return networkComputers;

        }
        #endregion
    }
    #endregion
}

   

using ListNetworkComputers;


        NetworkBrowser nb = new NetworkBrowser();

private void FormLogin_Load(object sender, EventArgs e)
        {
            try
            {
                foreach (string pc in nb.getNetworkComputers())
                {
                    comboBoxServer.Items.Add(pc);
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show("An error occurred trying to access the network computers", "error",
                                   MessageBoxButtons.OK, MessageBoxIcon.Error);
                Application.Exit();
            }

            
        }