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++

In reply to Re: Need a better way to break out a range of addresses... by bsdz
in thread Need a better way to break out a range of addresses... by RMaxwell

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.