It may be useful in certain circumstances (e.g. games) to open some ports on its box. For this is simple.
List of ports open on router or ISP box,
so that your communications software, P2P, server etc. ... working properly.
Each router has its specific interface; usually the settings are on the menu of your NAT router
Some software programs use specific ports, it is imperative to open to enable them to function normally
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