i know about port forwarding I want to open a lot of ports but there's a limited number of slots.
To answer your question. When you are behind a router you have a local address for your first computer- 192.168.1.100 second computer 192.168.101... and so on. A router comes with a firewall. So if you want applications that need internet access to fuction correctly you need to open a port for that specific app. And to do this is called forwarding, because the router sends information to the correct computer through the port. Well in my situation if I don't have enough slots for all my applications to connect to the internet or act as a server I need to enable dmz(which leaves my computer vunerable to hackers), which opens all ports.