vnix$ ./a.out 72.14.253.125 
in_addr: 72.14.253.125
po-in-f125.google.com

#include <netdb.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <errno.h>

int main (int argc, char *argv[])
{
  struct in_addr addr;
  struct hostent *he;
  /* printf ("sizeof struct in_addr: %i\n", sizeof(struct in_addr)); */
  printf ("in_addr: %s\n", argv[1]);
  addr.s_addr = inet_addr(argv[1]);
  if (addr.s_addr == 0 || addr.s_addr == -1)
    {
      perror ("inet_addr");
      return 1;
    }
  /* else */
  he = gethostbyaddr (&addr, 4, AF_INET);
  if (he == NULL)
    {
      perror ("gethostbyaddr");
      switch (h_errno)
	{
	case HOST_NOT_FOUND: printf ("HOST_NOT_FOUND\n"); break;
	case NO_ADDRESS: printf ("NO_ADDRESS\n"); break;
	  /* case NO_DATA: print ("NO_DATA\n"); break; */
	case NO_RECOVERY: printf ("NO_RECOVERY\n"); break;
	case TRY_AGAIN: printf ("TRY_AGAIN\n"); break;
	}
      return 1;
    }
  /* else */
  printf ("%s\n", he->h_name);
  return 0;
}