TWNIC_CIDR.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Net;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp2
{
    internal class Program
    {
        static void Main(string[] args)
        {
            Stopwatch sw = new Stopwatch();
            sw.Start();

            ShowCIDR();

            sw.Stop();
            Console.WriteLine($"Execution Time: {sw.ElapsedMilliseconds}ms");
            Console.ReadKey();
        }

        static void ShowCIDR()
        {
            var lines = File.ReadAllLines(@"C:\123123.txt");
            var rules = new List<string>();

            for (int i = 0; i < lines.Length; i++)
            {
                var line = lines[i].Split('\t');

                var ip_start = line[0];
                var range = int.Parse(line[2]);

            Loop:
                var ip_start_uint = IP2Uint(ip_start);
                var free_size = FreeSize(ip_start_uint);
                var pad_size = PaddingSize(free_size, range);
                var cidr = CIDR(ip_start, pad_size);
                rules.Add(cidr);

                if (pad_size < range)
                {
                    range -= pad_size;
                    ip_start = Uint2IP(ip_start_uint + (uint)(pad_size << 8));
                    goto Loop;
                }
            }
            Console.WriteLine(string.Join("\n", rules));
        }

        static uint IP2Uint(string addr)
        {
            var txt = addr.Split('.');
            var ip = (uint.Parse(txt[0]) << 24) + (uint.Parse(txt[1]) << 16) + (uint.Parse(txt[2]) << 8) + uint.Parse(txt[3]);
            return ip;
        }

        static string Uint2IP(uint addr)
        {
            return $"{addr >> 24 & 0xFF}.{addr >> 16 & 0xFF}.{addr >> 8 & 0xFF}.{addr & 0xFF}";
        }


        static int FreeSize(uint ip)
        {
            var size = 1;
            ip >>= 8;
            while ((ip & 1) == 0)
            {
                ip >>= 1;
                size <<= 1;
            }
            return size;
        }

        static int PaddingSize(int free, int range)
        {
            if (free <= range)
                return free;

            var i = 2;
            while (i <= range)
            {
                i <<= 1;
            }
            return i / 2;
        }

        static string CIDR(string ip, int size)
        {
            var cnt = 0;
            while (size > 1)
            {
                cnt++;
                size >>= 1;
            }
            return $"{ip}/{24 - cnt}";
        }
    }
}

https://rms.twnic.tw/help_ipv4_assign.php