#include <stdio.h>
#include <stdlib.h>

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <inttypes.h>

int main(int argc, char **argv)
{
	in_addr_t ip;   /* in network byte order */
	in_addr_t i;    /* in local byte order */
	char class;     /* IPv4 address class: A, B, C, D, or E */
	
	if (argc != 2) {
		fprintf(stderr, "%s: argument must be a dotted IPv4 address\n",
			argv[0]);
		return 1;
	}

	if (inet_pton(AF_INET, argv[1], &ip) != 1) {
		fprintf(stderr, "%s: argument %s must be a dotted IPv4 address\n",
			argv[0], argv[1]);
		return 2;
	}

	i = ntohl(ip);   /* network to host long */

	if ((i & 1 << 31) == 0) {
		class = 'A';   /* Bit 31 is 0 (includes loopback address). */
	} else if ((i & 1 << 30) == 0) {
		class = 'B';   /* Bit 31 is 1, bit 30 is 0. */
	} else if ((i & 1 << 29) == 0) {
		class = 'C';   /* Bit 31 is 1, bit 30 is 1, bit 29 is 0. */
	} else if ((i & 1 << 28) == 0) {
		class = 'D';   /* Bit 31 is 1, bit 30 is 1, 29 is 1, 28 is 0. */
	} else {
		class = 'E';   /* Bit 31 is 1, bit 30 is 1, 29 is 1, 28 is 1. */
	}

	printf("%s is a class %c IPv4 address.\n", argv[1], class);
	return EXIT_SUCCESS;
}