This is very useful in case you want to make services available from your router to the public. In the example I have shown destination NAT for FTP. Also don't forget to allow such traffic in the firewall - might get filtered out.
/ip firewall nat add chain=dstnat dst-address=<real ip> protocol=tcp dst-port=20-21 action=dst-nat to-addresses=<internal ip> to-port=20-21
/ip firewall nat add chain=dstnat dst-address=<real ip> protocol=tcp dst-port=20-21 action=dst-nat to-addresses=<internal ip> to-port=20-21
