Not able to download the content file - Getting Error: The proxy name could not be resolved: 'http'

I am trying to download the content file from specific URL. But, i'm  getting error.

below is my code : 

  DownloadURL = "http://servertest.mnet.comp.intranet:80/tm-rest/repos/contents/content";

                        // To get content from URL

                        WebClient client = new WebClient();
                        WebProxy wp = new WebProxy("http://servertest.mnet.comp.intranet",80);
                        wp.Credentials = new NetworkCredential("test1", "pass", "mnet.comp.intranet");
                        client.Proxy = wp;
                        client.DownloadFile(DownloadURL, "c:\\Content\\content.txt");

Error : The proxy name could not be resolved: 'http'

What is the error and how we could solve this problem...?