Request for the permission of type 'System.Net.DnsPermission, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed

hello,

Am trying to find user's ipadress using below code,

 IPHostEntry host;
                string localip = "";
                host = Dns.GetHostEntry(Dns.GetHostName());

                foreach (IPAddress ip in host.AddressList)
                {
                    if (ip.AddressFamily.ToString() == "InterNetwork")
                    {
                        localip = ip.ToString() + "-";
                    }
                }

but am getting the below exception.

Request for the permission of type 'System.Net.DnsPermission, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed

so i thought it must be the permission issue so i gave full trust level from web.config file. But still it showing the error. can any one tell me how to resolve this issue.