use strict;
use warnings;

sub parse_ipv4 {
   local *_ = \(@_ ? $_[0] : $_);
   return unpack('N', pack('C4', split(/\./)));
}

sub format_ipv4 {
   local *_ = \(@_ ? $_[0] : $_);
   return join('.', unpack('C4', pack('N', $_)));
}

my $member = parse_ipv4('10.20.40.183');
my $mask   = parse_ipv4('255.255.0.0');

my $first     = $member & $mask;
my $broadcast = $first | ~$mask;

# my $num_machines = $broadcast - $first + 1 - 2;
# which simplifies to:
my $num_machines = ~$mask + 1 - 2;

printf("first        = %s\n", format_ipv4($first));
printf("mask         = %s\n", format_ipv4($mask));
printf("broadcast    = %s\n", format_ipv4($broadcast));
printf("first usable = %s\n", format_ipv4($first+1));
printf("last  usable = %s\n", format_ipv4($broadcast-1));
printf("This subnet can handle %d machines:\n", $num_machines);

# 128.x.x.x and higher pack into a negative number on
# 32-bit machines, so the following won't work:
#
#    print(format_ipv4, $/)
#       for ($first+1 .. $broadcast-1);

if ($mask & 0x7FFFFFFF == 0) {
   print(format_ipv4($_), $/)
      for (1 .. 0x7FFFFFFF);
   print(format_ipv4($_|0x80000000), $/)
      for (0 .. 0x7FFFFFFE);
} else {
   print(format_ipv4($_+$first), $/)
      for ($first+1-$first .. $broadcast-1-$first);
}

__END__
first        = 10.20.0.0
mask         = 255.255.0.0
broadcast    = 10.20.255.255
first usable = 10.20.0.1
last  usable = 10.20.255.254
This subnet can handle 65534 machines:
10.20.0.1
10.20.0.2
.
.
.
10.20.255.253
10.20.255.254