I've downloaded and installed the DHCP server version 3.0 from ISC. It was installed on the Mandrake Linux 9.1. It's working fine, delivers IP configuration as necessary, but there is an interesting aspect - the addresses are being assigned starting from the highest one towards the lowest one. For example, if I create an address pool of X.Y.Z.1 - X.Y.Z.250, the first address being assigned is X.Y.Z.250, and not X.Y.Z.1, as it should be. How can I solve this issue?

Sorry, but i can not understand what is the issue.
A DHCP service dynamically delivers addresses - as You service does.
A client gets a unique address from the address pool and uses it. What specific address the client gets from a pool has no impact on the clients function.

But:
You could configure DHCPD and the clients to be "more sticky" on his address assignments. Chekc the manuals for "maximum lease time".

If you wnat to assign specific addresses to specific hosts (e.g. printserverbox), you need to add a specific "host" configuration directive for each system that should get a "fixed address" from the DHCPD.
There are lot's of samples and some quite good "HOWTO" documents out there ...

I mean, that a DHCP server should assign the addresses from the beginning of the pool, not from it's end as it does, i.e. last address in it. Once again, an example: There is a pool of 192.168.10.1 - 192.168.10.200. There are no "sticky" addresses. The "/etc/lib/dhcp/dhcp.leases" file is completely empty. A very first client trying to obtaing an IP address, receives the 192.168.10.200 address, and not the 192.168.10.1, as it should. The second client obtains the 192.168.10.199 address and so on.

I already understood You complaint from the 1st request.
I still can't understand, why the leases order has an importance for You application. Taking just the functional aspect (and by my experience since 1989), the adresses order really does not matter.

There is no rule for "ordered leases", and even no "MAY" expression within the written standards. Thus, a DHCP service may lease adresses from its pool in any order.

You probably should send Your request to the RFC editor. As far as i know, the RFC2131 (ftp://ftp.rfc-editor.org/in-notes/rfc2131.txt) does not contain any rules for the order of dynamic leases.

Another option would be to contact ISC (manufacturer of the ISC DHCPD) and ask them for the very reasons for "reverse ordered leases".