Re: Subnetting Class C Network

[sjones @ Aptech . com (Samuel D. Jones)]

I find the little C program included at the end useful.

Sam

> From firewalls-owner @
 GreatCircle .
 COM Mon Sep 30 22:03 PDT 1996
> X-Sender: pferguso @
 lint .
 cisco .
 com
> Mime-Version: 1.0
> Date: Tue, 01 Oct 1996 00:29:49 -0400
> To: "Charles_Ragan @
 ins .
 com" <Charles_Ragan @
 INS .
 COM>
> From: Paul Ferguson <pferguso @
 cisco .
 com>
> Subject: Re: Subnetting Class C Network
> Cc: "Harry Feltsadas" <harry @
 ns .
 fdc .
 nl>,
>         jfjohnm @
 ca-online .
 com (John McColley @ J F Engineering),
>         firewalls @
 GreatCircle .
 COM
> 
> Also, it goes without saying that classful routing protocols have outlived
> their usefulness, and should be abandoned at one's earliest convenience.
> 
> In fact, RIPv1 has been declared historical (or, rather, hysterical).
> 
> - paul
> 
> At 08:53 PM 9/30/96 -0500, Charles_Ragan @
 ins .
 com wrote:
> 
> >One other note, rfc 1878's recommendation allows for the usage of the first
> >and last subnet.  Routing protocols that carry subnet information in its
> >updates allow for this. Ones that don't (igrp, static, ripv1, etc.).  The
> >practice I follow is to use them last, if needed.
> >
> >Charles
> >
> 
> --
> Paul Ferguson                                           ||        ||
> Consulting Engineering                                  ||        ||
> Reston, Virginia   USA                                 ||||      ||||
> tel: +1.703.716.9538                               ..:||||||:..:||||||:..
> e-mail: pferguso @
 cisco .
 com                         c i s c o S y s t e m s
> 
> 


#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <ctype.h>

#define A 1
#define B 2
#define C 3

/* NOTE: Only class C is currently supported
*/

void dump_class_C(int b1, int b2, int b3, int b4, int sb)
{
   int s, h, sd, hd;

   sd = 2;
   for (s = 1; s < sb; s++)
      sd *= 2;
   hd = 2;
   for (h = 1; h < (8 - sb); h++)
      hd *= 2;

   printf("Network:  %d.%d.%d.%d/%d\n", b1, b2, b3, b4, 24 + sb);
   printf("Netmask:  %d.%d.%d.%d\n", 255, 255, 255, (sd - 1) << (8 - sb));

   for (s = 1; s < sd - 1; s++)
   {
      printf("\fSubnet:     %d", s);
      printf("\nNetwork:    %d.%d.%d.%d/%d", b1, b2, b3, s << (8 - sb), 24 + sb);
      printf("\nBroadcast:  %d.%d.%d.%d\n\n", b1, b2, b3, (s << (8 - sb)) + (hd - 1));
      for (h = 1; h < hd - 1; h++)
      {
         printf("    %d.%d.%d.%d\n", b1, b2, b3, (s << (8 - sb)) + h);
      }
   }
}

int main(int argc, char *argv[])
{
   char address[256], *cp;
   int class, sb, b1, b2, b3, b4;

   if (argc != 3)
   {
      printf("Usage:     subnet address subnet_bits\n\n");
      printf("Example:   subnet 197.34.16.0 3\n");
      return 1;
   }


   strcpy(address, argv[1]);

   cp = address;
   b1 = atoi(cp);

   while (isdigit(*cp))
      ++cp;
   if (*cp == '.')
      ++cp;
   else
   {
      printf("Bad address: %s\n", argv[1]);
      return 1;
   }

   b2 = atoi(cp);

   while (isdigit(*cp))
      ++cp;
   if (*cp == '.')
      ++cp;
   else
   {
      printf("Bad address: %s\n", argv[1]);
      return 1;
   }

   b3 = atoi(cp);

   while (isdigit(*cp))
      ++cp;
   if (*cp == '.')
      ++cp;
   else
   {
      printf("Bad address: %s\n", argv[1]);
      return 1;
   }

   b4 = atoi(cp);

   sb = atoi(argv[2]);

   if (b1 < 128)
      class = A;
   else if (b1 < 192)
      class = B;
   else if (b1 < 224)
      class = C;
   else
   {
      printf("Bad address (class out of range): %s\n", argv[1]);
      return 1;
   }

   switch (class)
   {
      case A:
         if (sb > 24)
         {
            printf("Too many subnet bits\n");
            return 1;
         }
         break;

      case B:
         if (sb > 16)
         {
            printf("Too many subnet bits\n");
            return 1;
         }
         break;

      case C:
         if (sb > 8)
         {
            printf("Too many subnet bits\n");
            return 1;
         }

         dump_class_C(b1, b2, b3, b4, sb);

         break;
   }

   return 0;
}