How is the port range divided?

0-1023 are well-known ports and need root (or setcap) to bind on Linux; 1024-49151 are registered with IANA; 49152 and above are ephemeral, which is where the system takes source ports for outgoing connections.

How do I find what is using a port?

macOS / Linux: lsof -i :3000 or ss -ltnp | grep 3000. Windows: netstat -ano | findstr :3000 for the PID, then tasklist | findstr PID. On macOS, port 5000 is often taken by AirPlay Receiver, which you can turn off in System Settings.

Does changing the default port improve security?

It cuts the noise from automated scanners but stops nothing targeted — nmap identifies the real service in seconds. What helps is binding to localhost or a private network, firewall allowlists, key-based login and actual authentication.

Why can my cloud server not send mail?

Most providers block outbound port 25 to fight spam. Use 587 (STARTTLS) or 465 (implicit TLS) against a mail provider, or ask support to unblock it.

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🔌Common Port Numbers - 60 Services With Security Notes

Firewall rules, a connection that will not open, a suspicious log line — all of them start with “what is this port for?”. This table is grouped the way you actually look things up: web, local development, remote access, databases, container ops, network services. More importantly it flags the ports that turn into an incident the moment they face the internet — Redis 6379, MongoDB 27017, Docker 2375 get found within hours.

Free · No sign-up · Runs in your browserLast updated: 2026-10-26

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How to use Port numbers

1Search by number (3306), service (redis) or purpose (remote desktop)
2Browse by group using the buttons
3Watch for the red “keep off the internet” markers

FAQ

How is the port range divided?: 0-1023 are well-known ports and need root (or setcap) to bind on Linux; 1024-49151 are registered with IANA; 49152 and above are ephemeral, which is where the system takes source ports for outgoing connections.
How do I find what is using a port?: macOS / Linux: lsof -i :3000 or ss -ltnp | grep 3000. Windows: netstat -ano | findstr :3000 for the PID, then tasklist | findstr PID. On macOS, port 5000 is often taken by AirPlay Receiver, which you can turn off in System Settings.
Does changing the default port improve security?: It cuts the noise from automated scanners but stops nothing targeted — nmap identifies the real service in seconds. What helps is binding to localhost or a private network, firewall allowlists, key-based login and actual authentication.
Why can my cloud server not send mail?: Most providers block outbound port 25 to fight spam. Use 587 (STARTTLS) or 465 (implicit TLS) against a mail provider, or ask support to unblock it.