Dynamically assign IP based on MAC

Hi all,

I understand how to assign static IP addresses with DHCP
based on MAC. But here's what I'd like to do: I'd like to dynamically
assign an IP address to a MAC based on some rule (the rule really
is: look up this MAC in a table and give the IP address from that
table,
unless it doesn't exist, in which case assign a new one and enter it
into the table). In other words, you'll get the same IP every time
you
connect with the same MAC, but we don't know all MACs beforehand.

.