Check the IP Address is valid or not.

// First we add Namespaces
 
using System.Data;
using System.Net;

protected void Button5_Click(object sender, EventArgs e)
{
        try
        {
            IPAddress address = IPAddress.Parse(TextBox1.Text);        //Check the IP address is Valid or Not
            Reponse.Write(" Valid IP address ");
        }
        catch(Exception ex)
        {
            Response.Write("incorrect IP address");
        }
}