Port 621 sits in the well-known port range with an official assignment, but you'll rarely find anything listening on it. It was registered for ESCP-IP (Epson Standard Code for Printers over IP), a protocol designed to let computers communicate with Epson printers over networks.1
The protocol never caught on. Most network printing moved to other standards—IPP on port 631, LPD on port 515, or vendor-specific protocols. Port 621 remains assigned in IANA's registry, but it's essentially abandoned.
What Range This Port Belongs To
Port 621 falls in the well-known port range (0-1023).2 These ports are controlled by IANA and assigned to specific services. Getting a well-known port requires going through IANA's formal assignment process—someone at Epson filed the paperwork, got the assignment, and the port has sat there ever since.
Well-known ports were meant to be stable, universal addresses for critical services. Port 80 for HTTP. Port 443 for HTTPS. Port 25 for email. But not every assignment worked out that way. Some protocols died. Some never launched. Some were replaced by better alternatives.
Port 621 is one of those—registered with good intentions, but bypassed by history.
Known Uses (Mostly Historical)
ESCP-IP: The official assignment. ESCP was Epson's printer command language—a way to tell dot-matrix and inkjet printers what to print. ESCP-IP was supposed to bring that protocol to networked printers. It didn't gain significant adoption.3
Confusion with FileMaker: Some sources incorrectly list port 621 as used by FileMaker Pro or macOS Server file sharing.1 This appears to be a documentation error. FileMaker Pro uses port 5003 for network file sharing.4
In practice, you're unlikely to find anything using port 621 unless you're working with very old Epson network printers.
Why Unassigned (or Barely Assigned) Ports Matter
The well-known port range is finite—only 1,024 addresses. Every protocol that got assigned a port in the early days of the Internet claimed a piece of that limited space. Some assignments are still essential (DNS on 53, SMTP on 25). Others are relics.
Port 621 is a reminder that the Internet's address space was allocated before anyone knew which protocols would survive. IANA assigned ports to services that seemed important at the time—and now many of those services are gone, but the ports remain reserved.
This matters because:
- The space is limited — Well-known ports are a finite resource, and some are wasted on dead protocols
- Security scanners look for them — Attackers scan well-known ports looking for services to exploit, even if those services are rarely used
- Admins need to know what's normal — If you see traffic on port 621, it's either legacy equipment or something unusual worth investigating
Checking What's Listening
If you want to see if anything on your system is using port 621:
On Linux or macOS:
On Windows:
Most likely, you'll find nothing. Port 621 is one of those assigned-but-empty addresses—reserved in the registry, unused in reality.
The Bigger Picture
The Internet is full of these ghosts. Protocols that made sense in 1985. Services that never launched. Standards that got replaced before they caught on. Port 621 is one of them—a reminder that not every assignment in IANA's registry corresponds to something you'll actually encounter.
When you see an unfamiliar port, it's worth checking whether it's actively used or just a historical artifact. Port 621 is the latter—assigned, documented, but in practice, forgotten.
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