private void BoxIP_TextChanged(object sender, TextChangedEventArgs e)

        {

            TextBox zBox = sender as TextBox;

            byte num;

            var cursor = zBox.Text.Length;

            if (!byte.TryParse(zBox.Text, out num))

            {

                System.Media.SystemSounds.Exclamation.Play();

                if (zBox.Text.Length < 2)

                {

                    zBox.Text = null;

                } else

                {

                   zBox.Text = zBox.Text.Remove(zBox.Text.Length - 1);

                   zBox.SelectionStart = cursor;

                   zBox.SelectionLength = 0;

                }

             }

        }