Port 10021 — Unassigned, But Not Unused

What This Port Is

Port 10021 has no official service assignment from IANA. It sits in the registered port range (1024-49151), which means it's available for use but not reserved for any specific protocol.¹

What Actually Uses It

Despite its lack of official designation, port 10021 appears in real infrastructure:

• Alternative FTP: Administrators sometimes configure FTP servers to listen on 10021 instead of the default port 21. This allows running multiple FTP instances on the same system or obscuring FTP services from casual scanning.²
• Device firmware updates: Network equipment like Ricoh multifunction devices have been observed using port 10021 for firmware delivery and updates.³

These are pragmatic choices, not standards. There's no RFC defining what 10021 should be. Someone needed a port, saw it was available, and used it.

Why Unassigned Ports Matter

The port system works because of its structure:

• 0-1023 (Well-Known): Reserved for system services—SSH at 22, HTTPS at 443, DNS at 53
• 1024-49151 (Registered): Available for official assignment, but can be used for anything if IANA hasn't claimed it
• 49152-65535 (Ephemeral): Operating systems use these for temporary client connections

Port 10021 is in the middle ground. It's far enough from system ports that it won't conflict with the OS, but organized enough that two systems can theoretically agree to use it for the same service. Sometimes that agreement never happens formally. Sometimes it emerges anyway.

Checking What Listens on 10021

On Unix/Linux:

sudo netstat -tulpn | grep 10021
# or
sudo ss -tulpn | grep 10021

On Windows:

netstat -ano | findstr 10021
# or
Get-NetTCPConnection -LocalPort 10021 -ErrorAction SilentlyContinue

On macOS:

sudo lsof -i :10021

If nothing listens there, that's the common state for most port numbers. 10021 is more often empty than occupied.

The Quiet Utility

Port 10021 represents something true about the Internet: not every decision needs a committee. When FTP administrators needed an alternative port, 10021 was sitting there. It was free. It was used. No RFC required.

The port number system works because most of it is this—practical, unformalized, sufficient. The famous ports at 22 and 443 get the headlines. Port 10021 just works, or doesn't, depending on what you're trying to do.