Why is this not working?

Im trying to get a list of available entries from ldap by simple filter: attribute cn must be less than 1000 and greater than 500. According to Net::LDAP documentation this could be done like this:

#!/usr/bin/env perl


use strict;
use warnings;

use Net::LDAP;

my $ldap = Net::LDAP->new('192.168.9.111');
my $mesg = $ldap->bind('cn=root,dc=lomonosov,dc=parallel,dc=ru', passw
+ord => 'rootpw');

$|=1;

print "Searching for uids less than 1000 and gt 500... ";

$mesg =
    $ldap->search(
        base => "ou=slurm,dc=lomonosov,dc=parallel,dc=ru",
        filter => "&(cn<=1000)(cn>=500)"
    );

$mesg->code && die $mesg->error;

print "Ok\nFound: ";

print join ", ", map $_->get_value("cn"), $mesg->entries;
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But it produces:

Searching for uids less than 1000 and gt 500... Ok
Found:
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I know that there is a two hundred or so entries in ldap for this request,

fisher% ldapsearch -h 192.168.9.111 -D "cn=root,dc=lomonosov,dc=parall
+el,dc=ru" -w "rootpw" -b "ou=slurm,dc=lomonosov,dc=parallel,dc=ru" "&
+(cn>600)(cn<1000)" |tail -3

# numResponses: 145
# numEntries: 144
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In fact, the problem experienced on a simple filters with 'less than' or 'greater than' comparisions; complex search filters like '|(cn=527)(cn=528)' works just fine. What am I doing wrong? How can I get a list of entries with said filter?

Answer found, thanks to all. The original request using ldapsearch was wrong (according to rfc4515 there isn't '<' sign but '<='); the server doesn't answer for that type of comparisions becouse of absence of index.