its like when you are behind a router with a private ip you cannot accept any incomming connections from the internet with out configuring your router for that.
becasue all the packets that leave the router goes with the source ip of the router's interface. so all of the PCs behind the router communicate with the net under that ip. router makes this process trance parent by doing stuff to the packet header when they pass through it. but when application need to accept incoming connections from the net. they can't do it cause, they are trying to connect to the router rather than the real pc which runs the service/app. so in order to solve this the routers do something called port mapping. which when configured will forward any packets that the router receives for a certain port to a specified machine in the LAN.
so as i run a lot of bittorrent clients and stuff they need to get incoming connections to run at the full speed. so i've add some entries. but when there are more entries there are more work for the router. so i suspected that this slows down the router, but im not sure about that.
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www.portforwarding.com 's guides... i am not a good explainer