How to snat a permenent public ip to a pppoe user whose gateway is 172.16.0.1 & ip is 172.16.0.111
/ip firewall nat
add action=src-nat chain=srcnat comment=“Source nat for 172.16.0.111” disabled=yes
src-address=172.16.0.111 to-addresses=
Replace with whatever IP address you add on to your public interface that you wish to forward to the user.
You may also want to add a dst-nat rule which is similar:
/ip firewall nat
add action=dst-nat chain=dstnat comment=“Destination nat for 172.16.0.111” disabled=yes
dst-address= to-addresses=172.16.0.111
Thanks alot for ur support 
Why source nat? Why not assign the public IP directly to the user in PPPoE?