public string GetUserIP()

   {

        string _userIP;

       if(Request.ServerVariables["HTTP_VIA"] == null)

         {

              userIP= Request.UserHostAddress;

       }

      else

        {

          userIP= Request.ServerVariables["HTTP_X_FORWARDED_FOR"];

      }

      return userIP;

  }

---------------------------------------------------------------------------

//下面这个比较全的 ，能够穿多层代理的，引用人的

    public static string IPAddress

    {

        get

        {

            string result = String.Empty;

            result = HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"];

            if (result != null && result != String.Empty)

            {

                //可能有代理

                if (result.IndexOf(".") == -1)    //没有“.”肯定是非IPv4格式

                    result = null;

                else

                {

                    if (result.IndexOf(",") != -1)

                    {

                        //有“,”，估计多个代理。取第一个不是内网的IP。

                        result = result.Replace(" ", "").Replace("'", "");

                        string[] temparyip = result.Split(",;".ToCharArray());

                        for (int i = 0; i < temparyip.Length; i++)

                        {

                            if ( IsIPAddress(temparyip[i])

                                && temparyip[i].Substring(0, 3) != "10."

                                && temparyip[i].Substring(0, 7) != "192.168"

                                && temparyip[i].Substring(0, 7) != "172.16.")

                            {

                                return temparyip[i];    //找到不是内网的地址

                            }

                        }

                    }

                    else if ( IsIPAddress(result)) //代理即是IP格式

                        return result;

                    else

                        result = null;    //代理中的内容 非IP，取IP

                }

            }

            string IpAddress = (HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"] != null && HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"] != String.Empty) ? HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"] : HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"];

            if (null == result || result == String.Empty)

                result = HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"];

            if (result == null || result == String.Empty)

                result = HttpContext.Current.Request.UserHostAddress;

            return result;

        }

    }

    public static bool IsIPAddress(string str1)

    {

        if (str1 == null || str1 == string.Empty || str1.Length < 7 || str1.Length > 15) return false;

        string regformat = @"^\d{1,3}[\.]\d{1,3}[\.]\d{1,3}[\.]\d{1,3}$";

        Regex regex = new Regex(regformat, RegexOptions.IgnoreCase);

        return regex.IsMatch(str1);