perl and network addresses

[Ben Scott]

On 3/27/06, Paul Lussier <p.lussier at comcast.net> wrote:
> I'm stumped.  I've got a network address space of 10.0.32/19.  How
> ever, this space is carved up using a /16 netmask.

  HUH?

> Given an address, say 10.0.33.189, I want to get the "network" and
> "host" portion of the address.

  (1) Red Hat provides a nifty utility called "ipcalc" that will do
that for you.  E.g.:

$ ipcalc -b -m -n -p 10.0.33.189/19
NETMASK=255.255.224.0
PREFIX=19
BROADCAST=10.0.63.255
NETWORK=10.0.32.0
$

  (2) I find it's a lot easier to conceptualize this stuff if you
write it out in binary notation (view using a fixed-width font):

/19 = 255.255.224.0

010.000.033.189 = 00001010 00000000 00100001 10111101
255.255.224.000 = 11111111 11111111 11100000 00000000
Net portion     = 00001010 00000000 00100000 00000000
Node portion    = 00000000 00000000 00000001 10111101

  (3) You mentioned Perl, but this is the same in most programming
languages: It's simple binary arithmetic and Boolean logic.  All you
need are AND and NOT (complement).  The trickier part is usually
converting from dotted-quad notation to binary storage.  Fortunately,
Unix provides a function for that: inet_addr(2).  C code looks like
this:

/* includes */
#include <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
/* main program */
int main() {
/* variables */
unsigned int a;  /* address */
unsigned int m;  /* mask */
unsigned int n;  /* node */
/* get address and mask, in network byte order */
a = inet_addr("192.0.2.42");
m = inet_addr("255.255.255.0");
/* find node portion by AND'ing with complement of net-mask */
n = a & (~m);
/* convert to host byte order */
n = ntohl(n);
/* print result as decimal integer */
printf("node=%d\n", n);
}

  I tried

	perl -we '$a = inet_addr("192.0.2.42");'

but it complained that inet_addr is not defined.  I suspect there's a
module somewhere you need to pull in.  Hopefully this is enough to get
you started.

-- Ben