newbies questions

[Pol Hallen]

Howdy :-)

First for all, I'm apologize for this questions, obviously I've read some
howto before post here.

I use debian 7

1) when is necessary clear the file dhcpd.leases? (/var/lib/dhcp/)

2) with a dhcp range (i.e.) 192.168.1.1 - 192.168.1.20, dhcp server does
an echo request for known if IP is available. If I've (with static IP) a
machine with IP 192.168.1.5 and iptables blocks echo requests, should I've
an IP conflicts?

3) last question :-) after dhcp assigns me an IP, if I change that IP and
put same IP of gateway I can crack whole network? How disallow this
situation?

thanks for replies :-)

Pol