Response.Write(Request.ServerVariables["Remote_Addr"]);

public string GetClientIPv4()
        {
            string ipv4 = String.Empty;
            foreach (IPAddress ip in Dns.GetHostAddresses(GetClientIP()))
            {
                if (ip.AddressFamily.ToString() == "InterNetwork")
                {
                    ipv4 = ip.ToString();
                    break;
                }
            }

            if (ipv4 != String.Empty)
            {
                return ipv4;
            }
            
            foreach (IPAddress ip in Dns.GetHostEntry(GetClientIP()).AddressList)
            {
                if (ip.AddressFamily.ToString() == "InterNetwork")
                {
                    ipv4 = ip.ToString();
                    break;
                }
            }
            return ipv4;
        }


        /// <summary>
        /// 取得客户端地址
        /// </summary>
        public string GetClientIP()
        {
            if (null == HttpContext.Current.Request.ServerVariables["HTTP_VIA"])
            {
                return HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"];
            }
            else
            {
                return HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"];
            }

        }