If interfaces are bridged then IP address should be set on bridge interface.
If there is an IP address assigned to interface that is configured as bridge port, that ip will be automatically assigned to bridge interface.
things is… do u have public static ip for all those 20-25 user first?
if u have u can bridge them and do dhcp on the bridge.. i think thats what u ask for distribution.
if u dont have more than 25 static ip..unfortunitly u have to do nat
its in ip/firewall/nat use the masquared to fake all those 25 ips.
u can also use dhcp on that way as well