注意使用多线程扫描需要为每个IP地址创建一个辅助线程
即为：

Thread t = new Thread(Scan);
                t.Start(str1);

核心代码：
 int count = 0;
        Stopwatch wall;
        private void Button_Click(object sender, RoutedEventArgs e)
        {            
            int start = int.Parse(a2.Text);
            int end = int.Parse(a3.Text);
            count = end - start+1;
            if(start>end)
            {
                MessageBox.Show("前地址大于后地址");
                return;
            }
            a5.Items.Clear();
            wall = Stopwatch.StartNew();
            string str = this.a1.Text;
            for (int i = start; i <=end; i++)
            {
                string str1 = str + i.ToString();
                Thread t = new Thread(Scan);
                t.Start(str1);

            }
            
           
        }
        int num;
        private object obj = new object();
        private void Scan(object ip)     //线程参数必须是object类型
        {
            num = 0;
            Stopwatch w = Stopwatch.StartNew();

            string host = "";
            try
            {
                host = Dns.GetHostEntry(ip.ToString()).HostName;
            }
            catch
            {
                host = "（不在线）";
            }
            w.Stop();

            a5.Dispatcher.Invoke(() => a5.Items.Add(
                     string.Format("扫描地址：{0}，扫描用时：{1}毫秒，主机DNS名称：{2}",
                     ip, w.ElapsedMilliseconds, host)));
            lock (obj)        //分配锁
            {
                num++;
            }
            if (num == count)
            {
                wall.Stop();
                a5.Dispatcher.Invoke(() => this.a5.Items.Add("all time:" + wall.ElapsedMilliseconds+"毫秒"));
            }

        }