A company that wants to connect to the Internet needs to find someone else that is willing to connect to them. Both parties will negotiate a deal to exchange traffic. For example, you may pay $1.00 to access the other party’s network, and they may offer you free access for a certain period of time.
After you have negotiated a deal, you will need to configure your router so that it knows about the new connection. Your router will tell all your devices that they now have access to the new connection, and all of your devices will automatically update their configuration files.
For example, HTTP is a popular web browser. HTTP uses port 80. HTTPS is a secure version of HTTP. HTTPS uses port 443. SMTP is a mail server. Mail servers use port 25. FTP is a file sharing program. File sharing programs use port 21.
More info: What is Domain Spoofing
