Tracking IP Address from Proxy Server

using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Web.ClientServices.Providers;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Net;
using System.Net.NetworkInformation;
using System.Net.Sockets;
public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
{
 private StringCollection GetIPs()
    {
        dynamic localIP = new StringCollection();
        dynamic localHostName = Dns.GetHostName();
        dynamic hostEntry = Dns.GetHostEntry(localHostName);
        // Grab all ip addesses.
        foreach (System.Net.IPAddress ipAddr in hostEntry.AddressList)
        {
            localIP.Add(ipAddr.ToString());
        }
        return localIP;
    }
}


}

Error 42 The type or namespace name 'dynamic' could not be found (are you missing a using directive or an assembly reference?)   
C:\Documents and Settings\Amul\Desktop\AJAXAMULabcd\STARTPAGE.aspx.cs  40  9   C:\...\AJAXAMULabcd\