#include <stdio.h>

#define CLASSA(addr)        (((unsigned long)(addr) & 0x80000000) == 0)
#define CLASSA_HOST(addr)    ((unsigned long)(addr) & 0x00ffffff)

#define CLASSB(addr)        (((unsigned long)(addr) & 0xc0000000) == 0x80000000)
#define CLASSB_HOST(addr)    ((unsigned long)(addr) & 0x0000ffff)

#define CLASSC(addr)        (((unsigned long)(addr) & 0xe0000000) == 0xc0000000)
#define CLASSC_HOST(addr)    ((unsigned long)(addr) & 0x000000ff)


unsigned long gethostfromaddr(unsigned long addr)
{
   printf("\nInternetadresse: %u \n", addr);
   if (CLASSA(addr))
   {
      printf("Class A\n");
      return  CLASSA_HOST(addr);
   }
   else if (CLASSB(addr))
   {
      printf("Class B\n");
      return CLASSB_HOST(addr);
   }
   else if (CLASSC(addr))
   {
      printf("Class C\n");
      return CLASSC_HOST(addr);
   }
   else 
   {
      printf("Falsche Angabe\n");
      
      /*(-1) wird durch den cast (...)in einen unsigned long umgewandelt,*/
      /*diese Zahl kann keinem Host entsprechen.                         */
      return (unsigned long) -1; 
   }
}

int main (void)
{
   unsigned long myaddr;
   printf("Internetadresse eingeben: ");
   scanf("%lu", &myaddr);
   printf("\nInternetadresse: %lu mit Host: %lu \n", myaddr, gethostfromaddr(myaddr));
   return 0;
}