Port 443 is the port number for the secure version of the Hypertext Transfer Protocol used to access web pages. It's used by servers and browsers to make sure that you access the right version of a site and that criminals aren't able to eavesdrop on you or tamper with the data you send across the internet. You can check if it's open on a given computer using your web browser or a more specialized tool.
How Ports Work
When it comes to internet connections, ports don't refer to physical outlets on computers but to virtual numbers used to identify different services running on a computer. They're used in conjunction with internet protocol, or IP, addresses to open connections between different devices.
An IP address is a unique identifier of a particular computer, and a port number identifies a type of software running on it. The IP address enables computer networks to route messages between machines and port numbers to let those computers route messages to the right programs. For example, port 25 is traditionally used for email services, port 80 is used for unencrypted HTTP, and port 443 is used for HTTPS.
Since port numbers are standardized, lots of software programs such as web browsers and email clients correctly use the default ports in most cases unless you specify otherwise.
Is Port 443 Open?
When a port is open, there's server software on a computer running and listening for connections to that port. If port 443 is open on a computer, that usually means web servers are waiting for a connection from a web browser.
You can test whether the port is open by attempting to open an HTTPS connection to the computer using its domain name or IP address. To do this, you type https://www.example.com in your web browser's URL bar, using the actual domain name of the server, or https://192.0.2.1, using the server's actual numeric IP address.
If some service other than a normal web server is using port 443, this process may fail. Look for an error message in your browser to understand exactly what's going on. You can also test whether port 443 is open using a tool designed to connect to specific ports on a computer. Two free ones are Netcat and Ncat.
Localhost and the 443 Port
In some cases, you may be testing software on your computer and want to see if you're able to connect to it using port 443 or another port.
As a shortcut, you can connect to your machine using the special domain name "localhost" or the special IP address 127.0.0.1. In some cases, software may allow a connection only from the same machine for security purposes.