dhcp addresses from pool only to specific devices

I have often set isc-dhcpd to give to each device its address, or to any 
device from a pool of addresses.
Now the number of "auathorized" devices has grown a lot (more than 300) 
but the pool is only about 90 addrresses. I know that is impossible that 
were active more than that number, and normally will be only around 40 
ones, but i cannot give a fixed address to each one, since i cannot know 
if a certain couple will be presetn at same time, so i have to use a pool, 
on the other side i need to give access only to machines known , so i have 
to resort on other system.
Looking at manual I should declare a class of allowed users (that is mac 
addresses) and allow only them to use a public address, while assinging to 
other ones a private one.
What is NOT clear to me is how to set a class including some hundreds mac 
addresses ... since all the examples I have seen use matches or other 
cases.
any hint ?
--
Leonardo Boselli
Università di Firenze
tel:+39 0552758808  +39 3488605348