Common TCP Ports Explained
A port is a numbered endpoint that lets one server run many services at once. TCP ports range from 0 to 65535; the well-known ones below 1024 are reserved for standard services. Knowing them makes connectivity problems much faster to diagnose.
Web and transfer
- 80 — HTTP (unencrypted web).
- 443 — HTTPS (encrypted web); the one that matters most today.
- 21 — FTP control (file transfer).
- 22 — SSH / SFTP (secure shell and secure file transfer).
- 23 — Telnet (legacy, insecure — avoid).
- 25 — SMTP (mail relay between servers).
- 587 — SMTP submission (clients sending mail, with auth).
- 465 — SMTPS (SMTP over TLS).
- 143 — IMAP (reading mail).
- 993 — IMAPS (IMAP over TLS).
- 110 / 995 — POP3 / POP3S.
Databases and infrastructure
- 3306 — MySQL / MariaDB.
- 5432 — PostgreSQL.
- 6379 — Redis.
- 27017 — MongoDB.
- 3389 — RDP (Windows remote desktop).
- 53 — DNS (uses both TCP and UDP).
Why check a port?
"The server is up but the service won't connect" almost always comes down to a port. A ping only proves the host answers; it says nothing about whether the specific service is listening. A TCP check on the exact port tells you whether that service is actually accepting connections.
A closed or filtered port usually means the service isn't running, is bound to the wrong interface, or a firewall is blocking it. Testing the port from outside your network confirms whether it's reachable from the internet at all.
Frequently asked questions
What's the difference between port 80 and 443?
Port 80 serves plain HTTP; port 443 serves HTTPS (HTTP over TLS). Modern sites use 443 so traffic is encrypted, and typically redirect 80 to 443.
How do I know if a port is open?
Run a TCP check against the host and port. If the connection is accepted, the port is open and the service is listening; if it times out or is refused, it's closed or filtered.