[Ottawa-pm] Fwd: CIDR to IP-netmask

[Mark Mielke]

I think it did get to the list - or I've seen it twice anyways. :-)

My personal take is that it is wrong to not use a module to do this, and 
my reasoning is that almost everybody who tries to do it themselves, I 
believe including the code you submitted, muck it up.

I had trouble reading the code, though, and not having time to analyze 
it for a particularly common mistake, I didn't bother responding... :-)

How do you handle:

127.1/24

Cheers,
mark


On 04/04/2011 09:38 PM, Allan Fields wrote:
> Dunno why this didn't get to the list.  It's not exactly off-topic.
>
> -Allan
>
> ---------- Forwarded message ----------
> From: allan.fields at gmail.com
> Date: Fri, 1 Apr 2011 19:19:54 +0000
> Subject: CIDR to IP:netmask
> To: ottawa-pm at pm.org
>
> What do you'all do to get the netmask in dotted-quad form from CIDR?
> Or apply the mask to your IP to get the subnet?
>
> Does anyone have the arithmetic answer?  No modules allowed!  I do a
> bitwise-stringy version.
>
> The reason I ask is I found this an interesting example of using
> perl's pack/unpack as an IP calculator.  Linux has a command named
> ipcalc, but you may know not all installs can just request specific
> binary/modules.
>
> As a reminder CIDR for IPv4 is:
>      IP ::= ([0-9]{1,3} '.' ){3} [0-9]{1,3}
>      MASK ::= integer(0..32)
>      CIDR ::= IP '/' MASK
>
> Example (not meant to resemble real network):
>      166.66.7.24/23
>      166.66.7.0/23
>      0.0.0.0/32
>      1.2.3.4/22
>
> I do something like:
>
> If ($IPADDR=~qr{ \s* ['"]*
>      ( (?:[0-9]{1,3}\.){3} [0-9]{1,3} ) /
>      (     [0-9]{1,2}.                           )
> ['"]* \s* }x) {
>      ($subnet_ip, $subnet_mask) = ($1,$2);
>
>      if (not defined $subnet_mask or
>          ($subnet_mask<  0 or $subnet_mask>  32)) {
>          print "CIDR: Invalid mask\n";
>          return 1;
>      }
>
>      my $bitstr = '1';
>            $bitstr.= '0' x (32 - $subnet_mask);
>
>      my $mask = join('.',unpack('C4',pack('B*',$bitstr)));
>
>      print "IP: $subnet_ip Netmask: $mask\n";
>
>      return 0;
> } else {
>      print "CIDR: Invalid\n";
>      return 1:
> }
>
> # More advanced examples include calculating the subnet mask for two
> CIDR's and determining if they reside in the same range.
>
> .. Tired of typing on my BB keyboard.
> Please post your code.
>
> Sent from my BlackBerry device on the Rogers Wireless Network
> _______________________________________________
> Ottawa-pm mailing list
> Ottawa-pm at pm.org
> http://mail.pm.org/mailman/listinfo/ottawa-pm


-- 
Mark Mielke<mark at mielke.cc>