Task: Canonicalize CIDR
C: #include <stdbool.h>
#include <stdio.h>
#include <stdint.h>

typedef struct cidr_tag {
    uint32_t address;
    unsigned int mask_length;
} cidr_t;



bool cidr_parse(const char* str, cidr_t* cidr) {
    int a, b, c, d, m;
    if (sscanf(str, "%d.%d.%d.%d/%d", &a, &b, &c, &d, &m) != 5)
        return false;
    if (m < 1 || m > 32
        || a < 0 || a > UINT8_MAX
        || b < 0 || b > UINT8_MAX
        || c < 0 || c > UINT8_MAX
        || d < 0 || d > UINT8_MAX)
        return false;
    uint32_t mask = ~((1 << (32 - m)) - 1);
    uint32_t address = (a << 24) + (b << 16) + (c << 8) + d;
    address &= mask;
    cidr->address = address;
    cidr->mask_length = m;
    return true;
}


void cidr_format(const cidr_t* cidr, char* str, size_t size) {
    uint32_t address = cidr->address;
    unsigned int d = address & UINT8_MAX;
    address >>= 8;
    unsigned int c = address & UINT8_MAX;
    address >>= 8;
    unsigned int b = address & UINT8_MAX;
    address >>= 8;
    unsigned int a = address & UINT8_MAX;
    snprintf(str, size, "%u.%u.%u.%u/%u", a, b, c, d,
             cidr->mask_length);
}

int main(int argc, char** argv) {
    const char* tests[] = {
        "87.70.141.1/22",
        "36.18.154.103/12",
        "62.62.197.11/29",
        "67.137.119.181/4",
        "161.214.74.21/24",
        "184.232.176.184/18"
    };
    for (int i = 0; i < sizeof(tests)/sizeof(tests[0]); ++i) {
        cidr_t cidr;
        if (cidr_parse(tests[i], &cidr)) {
            char out[32];
            cidr_format(&cidr, out, sizeof(out));
            printf("%-18s -> %s\n", tests[i], out);
        } else {
            fprintf(stderr, "%s: invalid CIDR\n", tests[i]);
        }
    }
    return 0;
}

C#: using System;
using System.Net;
using System.Linq;
                    
public class Program
{
    public static void Main()
    {
        string[] tests = {
            "87.70.141.1/22",
            "36.18.154.103/12",
            "62.62.197.11/29",
            "67.137.119.181/4",
            "161.214.74.21/24",
            "184.232.176.184/18"
        };
        
        foreach (string t in tests) Console.WriteLine($"{t}   =>   {Canonicalize(t)}");
    }
    
    static string Canonicalize(string cidr) => CIDR.Parse(cidr).Canonicalize().ToString();
}

readonly struct CIDR
{
    public readonly IPAddress ip;
    public readonly int length;
    
    public static CIDR Parse(string cidr)
    {
        string[] parts = cidr.Split('/');
        return new CIDR(IPAddress.Parse(parts[0]), int.Parse(parts[1]));
    }
    
    public CIDR(IPAddress ip, int length) => (this.ip, this.length) = (ip, length);
    
    public CIDR Canonicalize() =>
        new CIDR(
            new IPAddress(
                ToBytes(
                    ToInt(
                        ip.GetAddressBytes()
                    )
                    & ~((1 << (32 - length)) - 1)
                )
            ),
            length
        );
    
    private int ToInt(byte[] bytes) => bytes.Aggregate(0, (n, b) => (n << 8) | b);
    
    private byte[] ToBytes(int n)
    {
        byte[] bytes = new byte[4];
        for (int i = 3; i >= 0; i--) {
            bytes[i] = (byte)(n & 0xFF);
            n >>= 8;
        }
        return bytes;
    }
    
    public override string ToString() => $"{ip}/{length}";
}