Standard Port

In short: The officially assigned, generally known port for a particular service, as specified by IANA — e.g. 80 for HTTP or 22 for SSH.

In more detail: Standard ports mean that a client doesn’t have to guess on which port a service can be reached — it can rely on the convention. A service can still run on a different port (e.g. SSH on port 2222 instead of 22 for security reasons), but then has to be addressed explicitly by the client.

In Depth

Some common standard ports:

20/21  - FTP (data/control)
22     - SSH
23     - Telnet (unencrypted, outdated)
25     - SMTP
53     - DNS
80     - HTTP
110    - POP3
143    - IMAP
443    - HTTPS
3306   - MySQL
5432   - PostgreSQL

Standard ports are pure convention, not a technical requirement — an administrator can run any service on any free port. In practice this is used regularly nonetheless: running SSH on an unusual port instead of 22, for example, noticeably reduces the number of automated brute-force attempts by bots that reflexively only probe the known standard ports (“security through obscurity” — not full protection, but a sensible additional hurdle). Important internal services (databases, admin interfaces) should nevertheless never rely ONLY on a non-standard port as a protective measure, but should additionally be secured by real authentication and firewall rules.

See also: Port, Destination port