Another use common use of NAT is to take a small statically allocated block of addresses and map many computers into this smaller address space. This is easy to accomplish using what you already know about the map and portmap keywords by writing a rule like so:
map tun0 192.168.0.0/16 -> 20.20.20.0/24 portmap tcp/udp 20000:60000
Also, there may be instances where a remote application requires that multiple connections all come from the same IP address. We can help with these situations by telling NAT to statically map sessions from a host into the pool of addresses and work some magic to choose a port. This uses a the keyword map-block as follows:
map-block tun0 192.168.1.0/24 -> 20.20.20.0/24