Well in this case, you don't need to parse anything, you just need to get an HTTP request and get the stream, like this:

    HttpWebRequest req = (HttpWebRequest)WebRequest.Create("http://www.whatismyip.com/automation/n09230945.asp");
    using (StreamReader reader = new StreamReader(req.GetResponse().GetResponseStream()))
    {
        Console.WriteLine(reader.ReadToEnd());
    }

If you really want to use Html Agility Pack, this is the equivalent:

    HtmlWeb web = new HtmlWeb();
    Console.WriteLine(web.Load("http://www.whatismyip.com/automation/n09230945.asp").DocumentNode.OuterHtml);