Convert IPv4 range to list of CIDR prefixes

range2cidr.c

#include <arpa/inet.h>

#include <assert.h>
#include <err.h>
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <strings.h>

#define IPv4_BITS 32

static inline int min(int a, int b)
{
        return (a < b ? a : b);
}

static void
print_cidr(const char *start_ip, const char *end_ip)
{
        struct in_addr ip;
        uint32_t start, end;
        int zero_bits, bits;

        if (!inet_aton(start_ip, &ip))
                err(1, "inet_aton() for %s failed", start_ip);
        start = ntohl(ip.s_addr);

        if (!inet_aton(end_ip, &ip))
                err(1, "inet_aton() for %s failed", end_ip);
        end = ntohl(ip.s_addr);

        if(start > end)
                errx(1, "bad IP order: %s - %s", start_ip, end_ip);

        while (start <= end) {
                /* trailing zeros in IP determine max possible
                 * prefix size for this IP
                 */
                zero_bits = start == 0 ? IPv4_BITS : ffs(start) - 1;

                /* ip count in this prefix: 2^bits */
                bits = min(fls(end - start + 1) - 1, zero_bits);

                ip.s_addr = htonl(start);
                printf("%s/%d\n", inet_ntoa(ip), IPv4_BITS - bits);

                start += 1 << bits;
        }
        return;
}

int
main(int argc, char *argv[])
{
        /* check, that IP address fits into int */
        assert(sizeof(int) >= sizeof(uint32_t));

        printf("CIDR ranges for 192.168.0.1-192.168.255.254:\n");
        print_cidr("192.168.0.1", "192.168.255.254");
        printf("\n");

        printf("CIDR ranges for 0.0.0.1-255.255.255.254:\n");
        print_cidr("0.0.0.1", "255.255.255.254");

        exit(0);
}