Random port generator

When you start a local development server, a test database or a container, it needs a port that nothing else is using. Picking 3000 or 8080 every time leads to conflicts. This generator gives you a random port number outside the well-known range, so you can avoid collisions and avoid ports that need administrator rights.

How to use it

1. Generate a random port.
2. Check that it is free on your machine.
3. Use it in your server configuration, Docker port mapping or test setup.

Port ranges explained

• 0 to 1023: well-known ports. Reserved for standard services such as HTTP (80), HTTPS (443), SSH (22) and SMTP (25). Binding to them usually requires root or administrator privileges.
• 1024 to 49151: registered ports. Assigned by IANA to specific applications, such as PostgreSQL (5432), MySQL (3306) and Redis (6379), but usable by anyone.
• 49152 to 65535: dynamic or ephemeral ports. Used by the operating system for outgoing connections. Linux uses 32768 to 60999 by default.

Checking whether a port is free

• Linux and macOS: lsof -i :4821 or ss -ltn | grep 4821.
• Windows: netstat -ano | findstr :4821.

No output means nothing is listening on that port.

Tips

• Avoid ports used by common tools in your stack, such as 5432, 6379, 27017 and 9200, even if they are not running right now.
• In automated tests, the most reliable approach is to bind to port 0 and let the operating system pick a free port, then read the assigned number.
• Random ports in the ephemeral range can occasionally collide with outgoing connections; a port between 1024 and 32767 is usually safest for servers.
• Remember to open the port in your firewall if other machines need to connect.