What Port 1901 Is
Port 1901 is registered with IANA — the Internet Assigned Numbers Authority, the body that keeps the official list of port assignments — for a service called fjicl-tep-a: the Fujitsu ICL Terminal Emulator Program A.1
The registrant on record is Bob Lyon. The protocol covers both TCP and UDP.
You are almost certainly not running it on any machine you own.
The ICL Story
ICL — International Computers Limited — was Britain's national computing champion. Formed in 1968 by government mandate to create a British rival to IBM, ICL built mainframe systems that dominated European enterprise computing through the 1970s and 80s. Universities, government agencies, banks, and large corporations across the UK and Commonwealth ran ICL hardware.
Fujitsu acquired ICL in the 1990s as the company's fortunes declined. The ICL brand survived into the 2000s before being retired entirely.
The fjicl-tep-a protocol was terminal emulation software that allowed users to connect to ICL mainframes — the same basic concept as Telnet, but speaking ICL's proprietary dialect. Someone at Fujitsu ICL registered the port with IANA, presumably because organizations running the software needed a standardized port for firewall rules and network documentation.
The IANA registration outlasted the company, the mainframes, and the users.
What You'll Actually Find on Port 1901
Nothing official. In practice, port 1901 is squatted by whatever application needs a port and picks one:
- NextPVR, a home TV recording server, uses port 1901 for its UPnP/SSDP service — a side effect of sitting next to port 1900, the standard SSDP discovery port.2
- Some network management tools and proprietary enterprise software use it informally.
- Security databases note that at least one older piece of malware used port 1901 for command-and-control communication — a hazard of unmonitored registered-but-dormant ports.3
If you're seeing traffic on port 1901 on your network, it's almost certainly not an ICL mainframe.
How to Check What's Listening on Port 1901
On Linux or macOS:
On Windows:
The process ID in the output tells you which application claimed the port. Cross-reference with Task Manager or Get-Process in PowerShell.
Why Registered-but-Dormant Ports Matter
The IANA registered ports range (1024–49151) exists to prevent chaos — if every application just picked a random port, conflicts would be constant. When a vendor registers a port, they're reserving it so their users can configure firewalls and routers with stable, predictable rules.
But registrations don't expire. Ports stay registered long after the software dies, the company dissolves, or the protocol becomes obsolete. Port 1901 is one of hundreds of such ghosts in the IANA registry — officially claimed, practically vacant.
This matters for two reasons. First, firewall administrators sometimes block or flag ports based on IANA assignments, which means traffic to port 1901 might be treated as "Fujitsu ICL protocol" by security tools — a category that hasn't been relevant in decades. Second, the vacancy invites squatting: when no active service is using a registered port, other software fills the gap informally, creating the mismatches you see in practice.
The IANA registry is a historical document as much as a technical one. Port 1901 is a small data point from British computing history, preserved in a database most people never think about.
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