網域名稱IP查尋的小工具

主要代碼如下：
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Net;

namespace ip2domain
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            CenterToScreen();
        }

        public bool checkip(string IP)
        {

            string[] vList = IP.Split('.');

            if (vList.Length != 4) return false;

            for (int j = 0; j < vList.Length; j++)
            {

                int i;

                if (!int.TryParse(vList[j], out i))
                    return false;

                if ((i < 0) || (i > 254))
                    return false;

                if ((i == 0) && (j == 0))
                    return false;

            }

            return true;

        }

        IPHostEntry iphost;

        private void button_ok_Click(object sender, EventArgs e)
        {
            try
            {
                if (textBox_input.Text != "")
                {

                    listBox1.Items.Clear();
                    iphost = Dns.GetHostEntry(textBox_input.Text);
                    string hostname = iphost.HostName;
                    listBox1.Items.Add("主機名稱: " + hostname);
                    listBox1.Items.Add("");
                    for (int i = 0; i < iphost.AddressList.Length; i++)
                    {
                        IPEndPoint ipend = new IPEndPoint(iphost.AddressList[i], 0);
                        IPAddress ip = ipend.Address;
                        string ips = ip.ToString();
                        if (ips != "")
                        {
                            listBox1.Items.Add(ip);
                            listBox1.Items.Add("");
                        }
                        else
                        {
                            MessageBox.Show("No ip infomation!");
                        }
                    }
                }

                //else
                //{
                //    MessageBox.Show("There must be some wrong!Please retry!");

                //    textBox_input.Text = "";
                //}
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
                textBox_input.Text = "";
            }

        }

        private void button1_Click(object sender, EventArgs e)
        {
            listBox1.Items.Clear();
            bool ck = checkip(textBox_input.Text);
            if (!ck)
            {
                MessageBox.Show("您輸入有誤!");
                textBox_input.Text = "";
            }
            else
            {
                iphost = Dns.GetHostEntry(textBox_input.Text);
                listBox1.Items.Add(iphost.HostName);
            }
        }

        private void textBox_input_MouseEnter(object sender, EventArgs e)
        {
            textBox_input.SelectAll();
        }
    }
}