Hello,
I'm using my linux machine to be dhcp server.
Please, help me to configure it in a way it should not remember the clients, which have just disconnected from it.

The example of my problem is:
...server is up with IPs pool={IP1 ....IP100}
...client1 connects and gets IP100
...client2 connects gets IP99
...cleant1 disconnects
...client3 connects and gets IP98 !!!!!!!!!! My target is that the client3 should get the IP100, because this IP is the first IP-free in the pool.

The only way to solve this problem I've found, is to remove the dhcpd.leases file and to restart the dhspd. The change of dhcpd.conf of "max lease time and default lease time" doesn't change this behaviour.

Please give me you advise.

why would you want it to act like this? that's not the way that dhcp is meant to work. Each client will usually remember what it's last IP was, and ask for that same one with a DHCPREQUEST query to the DHCP server. As DHCP is dynamic, you should never try to build any systems around it, where you trying to maybe keep all existing connections in a single range of IP's...?