Which port is opened in the firewall (by default) to allow an application to communicate with an external web service? Is it 21, 25, 80 or 1433 ?
There are hundreds if not thousands of ports that might be used by any particular system or service.
When a browser talks to a web server, it will generally use port 80 for unencrypted communications and port 443 for SSL/TLS encrypted communications. But this isn't mandatory, you will sometimes see ports like 8080, 8000, 9000, 9090 to name just a few.
Port 21 is used by unencrypted file transfers (FTP) and Port 25 is used by mail servers (for sending mail over unencrypted SMTP connections).
For other applications, it is best to firstly check the system documentation and then double check in the configuration (since the ports can often be changed). Beyond that, there are online databases that will tell you the common uses of ports.
There are 65536 ports (0-65535) and a web service can use any one or more of these.
HTTP servers traditionally use port 80.
HTTPS servers traditionally use port 443.
However, these port numbers are not binding. Any protocol can run on any port. They are just official or unofficial guidelines.