Determine IPv4 class. Just a quick teaching tool for those of us who need to convince ourselves that the most significant bits thing works.

ipclass.c

/********************************************************
 * ipclass.c 
 * 
 * Demonstrate that the first significant bits
 * can be used to determine the address class 
 * 
 * 
 * https://en.wikipedia.org/wiki/Classful_network
 *******************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char **argv){

  /* 
   * define the starting point for our classes, made easier
   * to read with binary constants.
   * https://gcc.gnu.org/onlinedocs/gcc/Binary-constants.html
   */

  int A = 0b00000000; 		/* 0 */
  int B = 0b10000000;		/* 128 2^7 */
  int C = 0b11000000;		/* 190 2^7 + 2^6*/
  int D = 0b11100000;		/* 224 2^7 + 2^6 + 2^5 */
  int E = 0b11110000;		/* 240 2^7 + 2^6 + 2^5 + 2^4*/
  

  /* if no argument supplied, just print the starting point of each class
   *  and call it a day 
   */
  if (argc < 2){
    printf ("A\t%d.0.0.0\n", A);
    printf ("B\t%d.0.0.0\n", B);
    printf ("C\t%d.0.0.0\n", C);
    printf ("D\t%d.0.0.0\n", D);
    printf ("E\t%d.0.0.0\n", E);    
    return 0;
  }

  /* chop off input from first 'dot' */
  int i = 0;
  for ( i = 0; i < strlen(argv[1]); i++){
    if (argv[1][i] == '.') { argv[1][i] = '\0'; break; } 
  }

  /* convert chopped input to integer */
  int first_bits = atoi(argv[1]);

  /* determine the address class */
  char class;

  if ( first_bits < B){ class = 'A'; }
  else if (first_bits < C) { class =  'B'; }
  else if (first_bits < D) { class =  'C'; }
  else if (first_bits < E) { class =  'D'; }
  else { class = 'E'; }

  /* print the output */
  printf ("%c\n", class);

  /* fin. */
  return 0;
  
}