将 CIDR 掩码转换为 IP 掩码

Question

将 CIDR 子网掩码转换为 IP 掩码标准的最简单方法是什么？

假设CIDR中的掩码是

24

，那么我需要得到

255.255.255.0

.

Answer 1

using System;
using System.Net;

namespace test2
{
    class Program
    {
        static IPAddress CidrToMask(int cidr)
        {
            int shift = 32 - cidr;
            UInt32 mask;
            if (shift >= 32)
                mask = 0;
            else
                mask = (0xFFFFFFFF << shift) >> shift;

            return new IPAddress(mask);
        }

        static void Main(string[] args)
        {
            for (int i = 0; i <= 32; i++)
                Console.WriteLine("cidr {0}, mask {1}", i, CidrToMask(i).ToString());
        }
    }
}

结果输出：

cidr 0, mask 0.0.0.0
cidr 1, mask 1.0.0.0
cidr 2, mask 3.0.0.0
cidr 3, mask 7.0.0.0
cidr 4, mask 15.0.0.0
cidr 5, mask 31.0.0.0
cidr 6, mask 63.0.0.0
cidr 7, mask 127.0.0.0
cidr 8, mask 255.0.0.0
cidr 9, mask 255.1.0.0
cidr 10, mask 255.3.0.0
cidr 11, mask 255.7.0.0
cidr 12, mask 255.15.0.0
cidr 13, mask 255.31.0.0
cidr 14, mask 255.63.0.0
cidr 15, mask 255.127.0.0
cidr 16, mask 255.255.0.0
cidr 17, mask 255.255.1.0
cidr 18, mask 255.255.3.0
cidr 19, mask 255.255.7.0
cidr 20, mask 255.255.15.0
cidr 21, mask 255.255.31.0
cidr 22, mask 255.255.63.0
cidr 23, mask 255.255.127.0
cidr 24, mask 255.255.255.0
cidr 25, mask 255.255.255.1
cidr 26, mask 255.255.255.3
cidr 27, mask 255.255.255.7
cidr 28, mask 255.255.255.15
cidr 29, mask 255.255.255.31
cidr 30, mask 255.255.255.63
cidr 31, mask 255.255.255.127
cidr 32, mask 255.255.255.255

Answer 2

public static string CidrToMask(int cidr)
{
    int shift = 32 - cidr;
    UInt32 mask;
    if (shift >= 32)
        mask = 0;
    else
        mask = (0xFFFFFFFF << shift) >> shift;

    return new IPAddress(mask).ToString();
}

public static int MaskToCdir(string subnetmask)
{
    var dict = new Dictionary<string,int>();
    for (var i = 0; i <= 32; i++)
        dict.Add(CidrToMask(i),i);
    if (dict.ContainsKey(subnetmask))
        return dict[subnetmask];
    return 0;
}