help on assigning invalid IP addresses

subnet with some server (fixed public IP addresses), workstation 
(same, assigned by dhcp), portable (dhcp dynamic), peripherals using only 
link local fe80:: IPv6 
addresses.
All was fine until arrived a couple of peripherals that get their IPv4 
address via dhcp and there is no way to avoid them to get it, even if you 
access then only via IPv6 .
I have added in the imcluded file that assign always the same address to 
workstations based on mac addresses record as

host XRX0000AABBCCDD {
    hardware ethernet 00:00:aa:bb:cc:dd;
    fixed-address 10.240.13.112;
    option routers 10.240.13.1;
    option subnet-mask 255.255.255.0;
    }

but the resut is that while addresse in the "real" subnet assigned to 
workstations works fine an are correctly assigned, peripherals insist on 
getting public addresses from the pool of portables, that is something i 
do not want.
How can i assign completely invalid (or better, unrouted) IPv4 addresses ?

--
Leonardo Boselli
DICEA
tel +39 0552758808  +39 3488605348