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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 ?

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Are you asking how data is returned when you are browsing web page or is this a specific application communicating with a server in some way. More info would assist. Most firewalls use several ways to manage traffic(stateful inspection for one) to allow traffic without user configuration –  Dave M Jan 30 at 17:29
    
It entirely depends on the host. The standard is port 80 and 443 for SSL traffic. –  Ramhound Jan 30 at 17:45
    
I agree with @DaveM. Are you a client trying to communicate through a firewall or trying to run a server and accept external connections? This is not clear from the question. What type of web service are you asking about? A very comprehensive list of the common TCP and UDP port numbers is given on Wikipedia although a service can use any port. What is the problem you are having? –  zelanix Jan 30 at 17:46
    
For reference, the ports you listed are generally: 21 - FTP, 25 - SMTP (sending mail), 80 - HTTP (web requests), and 1433 is generally MySQL. You haven't made it clear if you want to open listening ports on your server or outgoing ports on your clients. –  ernie Jan 30 at 17:52
    
@zelanix Suppose, it is an application that you have created in microsoft visual studio. And this application is required to communicate with an external web service. –  Shanks Jan 30 at 18:24

2 Answers 2

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.

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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.

Here are traditionally used port numbers for other popular services.

However, these port numbers are not binding. Any protocol can run on any port. They are just official or unofficial guidelines.

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