在Asp.Net Core中使用IP范围进行CORS配置，可以使用自定义中间件来实现。以下是一个示例代码：

首先创建一个名为CorsMiddleware的自定义中间件类：

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.Logging;
using System;
using System.Linq;
using System.Net;
using System.Threading.Tasks;

public class CorsMiddleware
{
    private readonly RequestDelegate _next;
    private readonly IConfiguration _configuration;
    private readonly ILogger _logger;

    public CorsMiddleware(RequestDelegate next, IConfiguration configuration, ILogger logger)
    {
        _next = next;
        _configuration = configuration;
        _logger = logger;
    }

    public async Task Invoke(HttpContext context)
    {
        var allowedIPRanges = _configuration.GetSection("AllowedIPRanges").Get();

        var clientIpAddress = context.Connection.RemoteIpAddress;

        if (clientIpAddress != null)
        {
            var isAllowed = allowedIPRanges.Any(ipRange => IPAddressRange.Parse(ipRange).Contains(clientIpAddress));
            if (!isAllowed)
            {
                _logger.LogWarning($"Request from IP address {clientIpAddress} blocked by CORS policy.");
                context.Response.StatusCode = (int)HttpStatusCode.Forbidden;
                return;
            }
        }

        await _next(context);
    }
}

然后，在Startup.cs文件的Configure方法中添加中间件：

public void Configure(IApplicationBuilder app, IWebHostEnvironment env, ILoggerFactory loggerFactory)
{
    // ...

    app.UseMiddleware();

    // ...
}

接下来，在appsettings.json文件中添加一个名为AllowedIPRanges的配置项，配置允许的IP范围：

{
  "AllowedIPRanges": [
    "192.168.0.0/24",
    "10.0.0.0/8"
  ]
}

这样，CORS中间件将会根据配置的IP范围来允许或拒绝请求。如果请求的IP地址不在允许的范围内，将返回HTTP状态码403 Forbidden。

请注意，为了使此示例代码正常工作，您需要添加Microsoft.Extensions.Configuration和Microsoft.Extensions.Logging包的引用。