I think on ISP should do at least these settings in order to minimize IP
from its network
If I have a network like 199.34.39.x, like this:
local network ----| eht0 ROUTER s0|----INET
then I setup my router to allow only the 199.34.39.x addresses thru its eth0
face and block packets coming in thru its s0 interface from the 199.34.39.x
Optionally, you can disable the network and broadcast addresses too.