An implementation:

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

[download]

Update: Tweaks.

Update 2: Fixed for ($first+1 .. $broadcast-1).

Note: Requires a perl with 32-bit integers.

Note: I didn't write code to check if IP address and the mask are in the valid format.

sub parse_ipv4 {
   local *_ = \(@_ ? $_[0] : $_);

   /^([0-9]+)\.([0-9]+)\.([0-9]+)\.([0-9]+)$/
   && $1 < 256
   && $2 < 256
   && $3 < 256
   && $4 < 256
   or return undef;

   return unpack('N', pack('C4', $1, $2, $3, $4));
}

[download]

Note: The mask isn't validated. The binary representation of the mask must matched (length == 32 && /^1*0*$/) || $_ == 0.

Note: Keep in mind the above code can print over 4 billion addresses!

    Mask            Number of addresses printed
/32 255.255.255.255 0
/31 255.255.255.254 0
/30 255.255.255.252 2
/29 255.255.255.248 6
/28 255.255.255.240 14
/27 255.255.255.240 30
/26 255.255.255.240 62
/25 255.255.255.240 126
/24 255.255.255.0   254
...
/16 255.255.0.0     65,534
...
/8  255.0.0.0       16,777,214
...
/3  224.0.0.0       536,870,910  
/2  192.0.0.0       1,073,741,822
/1  128.0.0.0       2,147,483,646
/0  0.0.0.0         4,294,967,294

[download]

In reply to Enumerating IP Addresses in a Subnet by ikegami
in thread algorithm for network address enumeration by knave

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