ユーザの実際のIPアドレスを取得する

/// <summary>
        ///  IP 
        /// </summary>
        /// <returns></returns>
        public static string GetUserIp()
        {
            string ip = HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"];
            if (ip == null || ip.Length == 0 || ip.ToLower().IndexOf("unknown") > -1)
            {
                ip = HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"];
            }
            else
            {
                if (ip.IndexOf(',') > -1)
                {
                    ip = ip.Substring(0, ip.IndexOf(','));
                }
                if (ip.IndexOf(';') > -1)
                {
                    ip = ip.Substring(0, ip.IndexOf(';'));
                }
            }

            Regex regex = new Regex("[^0-9.]");
            if (ip == null || ip.Length == 0 || regex.IsMatch(ip))
            {
                ip = HttpContext.Current.Request.UserHostAddress;
                if (ip == null || ip.Length == 0 || regex.IsMatch(ip))
                {
                    ip = "0.0.0.0";
                }
            }
            return ip;
        }