A dedicated server, particularly one running Linux, has a standard set of ports that are configured to operate with certain applications. By default, most firewalls, even when they are secured, allow the default ports to send and/or receive information to and from the outside world. It is still important to know what their purposes are for both security and troubleshooting.
Without further explanation, here are some of the default Linux ports available on most servers, listed in no particular order.
80 – This is probably the most essential port on a web hosting server since all websites use it, whether you have one or one thousand. It is the standard HTTP port on all servers, and Apache HTTP Server uses it by default.
443 – Anytime someone connects to your site with HTTPS, they will use port 443. It is the default port for SSL encrypted sites.
25 – When a mail server, such as Sendmail or Postfix, sends email, this is the port it uses by default. Changing it may be a serious consideration if your ISP blocks port 25 in an attempt to derail spammers.
110 – Incoming mail from a POP3 mail server, such as Dovecot, will use port 110.
143 – Incoming mail that relies on IMAP instead of POP3, will use this port.
3306 – The port MySQL uses to connect to remote clients. If you manipulate your databases directly on the server, it is best to configure your firewall to keep this port closed.
21 – The standard port used for File Transfer Protocol (FTP).
22 – The standard port used for Secure Shell (SSH).
There are other possible ports you may use, such as control panels, game servers, and instant messaging. Check the specific software’s documentation for more information.