This is useful if you want to allow some services trough the router. There is also a DST nat have a look at this post as well: http://yuriystanchev.blogspot.com/2014/01/mikrotik-destination-nat.html
/ip firewall nat add chain=srcnat src-address=<internal ip> protocol=tcp dst-port=<port> action=src-nat to-addresses=<real ip> to-port=<port>
Have fun.
/ip firewall nat add chain=srcnat src-address=<internal ip> protocol=tcp dst-port=<port> action=src-nat to-addresses=<real ip> to-port=<port>
Have fun.
