Split a range of IP addresses into subnets

ip-range-to-nets.c

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <arpa/inet.h>
#include <errno.h>
#include <string.h>

static void
print_cidr(uint32_t addr, int mask)
{
	uint32_t tmp_addr;
	char s[INET6_ADDRSTRLEN + 1];

	tmp_addr = htobe32(addr);
	inet_ntop(AF_INET, &tmp_addr, s, INET_ADDRSTRLEN);
	printf("%s/%d\n", s, mask);
}

int
main(int argc, char *argv[])
{
	uint32_t start, end;
	uint32_t subnet_first, subnet_last;
	uint32_t tmp_addr;
	int rc;

	/* processing command line */
	if (argc < 3) {
		fprintf(stderr, "Usage: %s IP1 IP2\n", argv[0]);
		return EXIT_FAILURE;
	}

	rc = inet_pton(AF_INET, argv[1], &tmp_addr);
	if (rc != 1) {
		fprintf(stderr, "Can't parse address '%s': %s\n",
			argv[1], strerror(errno));
		return EXIT_FAILURE;
	}
	start = be32toh(tmp_addr);

	rc = inet_pton(AF_INET, argv[2], &tmp_addr);
	if (rc != 1) {
		fprintf(stderr, "Can't parse address '%s': %s\n",
			argv[2], strerror(errno));
		return EXIT_FAILURE;
	}
	end = be32toh(tmp_addr);

	/* IP range is split into several subnets in loop */
	subnet_first = start;
	for (;;) {
		if (subnet_first > end) {
			break;
		} else if (subnet_first == end) {
			print_cidr(subnet_first, 32);
			break;
		}

		/* calculate last address in subnet
		 * for example if subnet_first == 192.168.1.0 last address
		 * (subnet_last) will be 192.168.1.255
		 */
		int mask_bits = __builtin_ctz(subnet_first);
		subnet_last = subnet_first + (1 << mask_bits) - 1;

		if (subnet_last == end) {
			/* end of subnet is exactly the end of user requested
			 * range, for example 192.168.1.255 == 192.168.1.255 */
			print_cidr(subnet_first, 32 - mask_bits);
			break;
		} else if (subnet_last > end) {
			/* user requested range is less then subnet
			 * say if user needs 192.168.1.0 - 192.168.1.88,
			 * but subnet is 192.168.1.0 - 192.168.1.255 */

			/* calculate difference between addressed first,
			 * for our example it will be (88 - 0 + 1) */
			uint32_t diff = end - subnet_first + 1;
			/* get mask of smaller subnet that fits in range
			 * for our example subnet will be 192.168.1.64/28 */
			int p =  32 - __builtin_clz(diff) - 1;
			/* get size of smaller subnet (64) */
			uint32_t ndiff = 1 << p;

			print_cidr(subnet_first, 32 - p);

			/* continue with adjusted range
			 * 192.168.1.64-192.168.1.88 */
			subnet_first += ndiff;
		} else {
			/* user requested range is bigger then subnet */
			print_cidr(subnet_first, 32 - mask_bits);

			/* continue with adjusted range */
			subnet_first = subnet_last + 1;
		}
	}

	return EXIT_SUCCESS;
}