Edit2：如上面提供的注释和链接中所述，HTTP_X_FORWARDED_FOR头可能包含多个以逗号分隔的IP地址。我没有面对这种情况，但假设需要对我的回答进行一些更正。

我使用这个代码获取IP地址(它返回IPAddress.None值如果由于某些原因失败)：

/// <summary>
    /// Gets the IP address of the request.
    /// <remarks>
    /// This method is more useful than built in because in some cases it may show real user IP address even under proxy.
    /// <summary>
    /// Gets the IP address of the request.
    /// <remarks>
    /// This method is more useful than built in because in some cases it may show real user IP address even under proxy.
    /// The <see cref="System.Net.IPAddress.None" /> value will be returned if getting is Failed.
    /// </remarks>
    /// </summary>
    /// <param name="request">The HTTP request object.</param>
    /// <returns></returns>
    public static IPAddress GetIp(this HttpRequest request)
    {
        string ipString;
        if (string.IsNullOrEmpty(request.ServerVariables["HTTP_X_FORWARDED_FOR"]))
        {
            ipString = request.ServerVariables["REMOTE_ADDR"];
        }
        else
        {
            ipString = request.ServerVariables["HTTP_X_FORWARDED_FOR"].Split(",".tocharArray(),StringSplitOptions.RemoveEmptyEntries)
                .FirstOrDefault();
        }

        IPAddress result;
        if (!IPAddress.TryParse(ipString,out result))
        {
            result = IPAddress.None;
        }

        return result;
    }