its not about the port you use, its about what device you tell your router you want that port to work for.
step 2 man! step 2!

right click yer network and go to status

the IP4 number is your computer's identification for using that router. every device that ever connects is gonna have a slightly different number.
when port forwarding, you use that number (or the last 3 digits as the rest should be there already) to designate what device uses that port.

this comp was given the number 103. so all ports for things i need to play from this comp, need to be forwarded to .103