I am not sure I understand the problem you are trying to solve, but if you just want to figure out if a given ip address is within a subnet it is easier if you convert the ip address, and network addresses and broadcast addresses from a quad 4 representation to an integer. Then do something like this ... 
#!/usr/bin/perl -w
use Socket;
use strict;

my %subnets = (
                '10.96.2.0/255.255.254.0'       => { 
                        start => 174064128,     
                        end => 174064639        }
        ,       '10.123.50.0/255.255.255.0'     => { 
                        start => 175845888,     
                        end => 175846143        }
        ,       '72.24.196.0/255.255.255.0'     => { 
                        start => 1209582592,    
                        end => 1209582847       }
        ,       '72.24.137.192/255.255.255.192' => { 
                        start => 1209567680,    
                        end => 1209567743       }
        ,       '10.122.50.0/255.255.255.0'     => { 
                        start => 175780352,     
                        end => 175780607        }
        ,       '65.181.207.128/255.255.255.128' => { 
                        start => 1102434176,    
                        end => 1102434303       }
);

my $ip = shift;
my $int = unpack("N",inet_aton(shift||$ip));
for my $subnet ( keys %subnets ) {
        if (    $subnets{$subnet}{'start'} <= $int &&
                $int <= $subnets{$subnet}{'end'}
        ) {
                print "$ip ($int) is in subnet $subnet (" . 
                        $subnets{$subnet}{'start'} . 
                        ", " . $subnets{$subnet}{'end'} . 
                        ") \n";
        }
}

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I used ipcalc to work out the network address and broadcast address and script I wrote, quad2int.pl, to convert those IP to integer.

In reply to Re: finding netmask for "arbitrary" ip address by Plankton
in thread finding netmask for "arbitrary" ip address by jasonl

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