As long as you don't mean blindingly fast and huge. You want Net::Netmask. 

#!/usr/bin/env perl
use strict;
use warnings;

use Net::Netmask;

# our owners and their ranges
my @blocks = (
    [ 'bob', '192.168.1.0/24' ],
    [ 'nancy', '192.168.1.12' ],
    [ 'ralph', '192.168.2.0/255.255.255.0' ],
    [ 'bob', '192.168.3.0/24' ],
    [ 'NOBODY', '0.0.0.0/0' ],
);

# populate the table with tagged blocks
for my $block (@blocks) {
    my ($owner, $range) = @$block;
    my $nb = Net::Netmask->new2($range);
    $nb->tag('owner', $owner);
    $nb->storeNetblock();
};

# do the lookup.
while (my $ip = <DATA>) {
    chomp $ip;
    my $block = findNetblock($ip);
    print "$ip belongs to ", $block->tag('owner'), "\n";
}

__DATA__
192.168.1.45
192.168.1.12
192.168.2.4
192.168.3.33
192.168.4.12

[download]
Output: 
$ ./net_netmask.pl 
192.168.1.45 belongs to bob
192.168.1.12 belongs to nancy
192.168.2.4 belongs to ralph
192.168.3.33 belongs to bob
192.168.4.12 belongs to NOBODY

[download]
This works rather well unless you're trying to grok the full internet route tables or such. (Somewhere there's a XS module that implements the routing table from BSD if you need really fast/big).

In reply to Re: Quickly determine which IP range an IP belongs to. by Anonymous Monk
in thread Quickly determine which IP range an IP belongs to. by punch_card_don

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.