You can use following piece of code and see if it helps
string[] hostEntries = System.Net.Dns.GetHostEntry(Request.ServerVariables["remote_addr"]).HostName.Split(new Char[] { '.' });
txtECN.Text = hostEntries [0].ToString();
However, not sure how it will behave in case the user is "behind" firewall/proxy.
Hope that helps
Milind