Wow! I think I missed this one :(

Still, here is yet another alternative using Perl out of the box. No doubt, a master monk can probably shorten it even more.

use strict;
use warnings;

my @a = map { s/\s*(#.*)?$//g; $_ } <DATA>;

my @b;
while (local $_ = shift @a) {
    if (m/\D(\d+)-(\d+)(?:\D|$)/) {
        my $range = "$1-$2";
        foreach my $i ($1..$2) {
            local $_ = $_;
            s/(\D)$range(\D|$)/$1$i$2/;
            push @a, $_;
        }    
    }
    else {
        push @b, $_;
    }
}

print join("\n", @b);

__END__
172.17.119.2 # Comments are here...
172.17.119.4-5 # Comments are here...
172.19-21.254.2-3 # Comments are here...
192.168.1.1-3 # Comments are here...
[download]

Update: Slightly saner while loop using localized stack.
Update 2: Removed unnecessary localization of $1, $2. Thanks to ikegami++