Only deals with IPv4.. but extending it wouldn't be hard.

#! perl -slw
use strict;

our $M ||= 30; our $IP ||= '1.2.3.4';

sub bin2dd{ join '.', unpack 'C4', pack 'N', $_[0] }
sub dd2bin{ unpack 'N', pack'C4', split'\.', $_[0] }

sub CIDRList{
    my $iter = 0xffffffff >> $_[0];
    my $mask = ~$iter;
    my $lo = dd2bin( $_[1] ) & $mask;
    map{ bin2dd $lo++ } 0 .. $iter;
}

print for CIDRList $M, $IP;

__END__
P:\test>CIDR -M=28 -IP=64.96.128.0
64.96.128.0
64.96.128.1
64.96.128.2
64.96.128.3
64.96.128.4
64.96.128.5
64.96.128.6
64.96.128.7
64.96.128.8
64.96.128.9
64.96.128.10
64.96.128.11
64.96.128.12
64.96.128.13
64.96.128.14
64.96.128.15
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