URL Ports
The port in a URL is a virtual point where network connections are made. Ports are managed by a computer's operating system and are numbered from 0 to 65,535 (Though port 0 is reserved for the system API).
Whenever you connect to another computer over a network, you're connecting to a specific port on that computer, which is listened to by a program on that computer. A port can only be used by one program at a time, which is why there are so many possible ports.
The port component of a URL is often not visible when browsing normal sites on the internet, because 99% of the time you're using the default ports for the HTTP and HTTPS schemes: 80 and 443 respectively.
Whenever you aren't using a default port, you need to specify it in the URL. For example, port 8080 is often used by web developers when they're running their server in "test mode" on their own machines.
If a port isn't present in a URL the default is used based on the...