Port 191 (TCP/UDP) is officially assigned to the Prospero Directory Service, a distributed file system protocol from the early 1990s. The protocol is no longer in use. Almost no Prospero servers exist today.
What Was Prospero?
Prospero was a distributed file system built on the Virtual System Model, designed by B. Clifford Neuman at the University of Washington.1 The core idea: let users create their own customized views of Internet resources, organizing files and services in whatever structure made sense to them.
In Prospero, all names for an object were primary. An object could physically reside anywhere, but users could access it through whatever naming structure they built. You constructed your own virtual system by selecting and organizing the objects and services that mattered to you.2
This was radical in the early Internet. Before search engines homogenized how we find things, Prospero imagined a world where everyone organized the Internet differently—where your view of available resources could be completely different from mine, and both could be valid.
How It Worked
The Prospero Directory Service was a name server that provided object-centered naming—naming without a global context. Instead of a single hierarchical structure everyone had to navigate, Prospero let you build your own structure.3
Prospero URLs (prospero://host:191/path) worked by contacting a Prospero directory server on port 191 to determine appropriate access methods for a resource. The directory service would resolve the name and tell your client how to access the actual file or service.
The system extended beyond just file access. Authentication, authorization, and service selection were all affected by your active virtual system. Your customized view determined what you could see and how you could interact with it.
Why It Died
Prospero was not widely implemented.4 The Web won. HTTP and HTML provided a simpler model: one global namespace, one way to link things together, and browsers that showed everyone the same view of a page. Search engines like Google later made the idea of customized directory structures seem unnecessary—why organize resources yourself when a search box can find anything instantly?
But something was lost. Prospero's vision was more personal, more adaptable. It assumed users would want different structures, different ways of seeing the same information. The Web assumed everyone should see the same thing.
Well-Known Port Status
Port 191 sits in the well-known ports range (0-1023), reserved by IANA for standardized services that system processes provide.5 Getting a well-known port number meant Prospero was considered significant enough to reserve permanently.
That assignment still exists. Port 191 is still officially allocated to Prospero, decades after the last server went offline. The port number is a fossil—a reminder that the Internet could have been organized differently.
Current Use
There is no significant current use of port 191. Some historical documentation mentions potential trojan or virus activity using this port, but there's no evidence of widespread malicious use.6
If you see traffic on port 191, it's either:
- A misconfigured service using the wrong port
- Someone experimenting with the historical Prospero protocol
- Malicious activity deliberately choosing an obscure port number
To check what's listening on port 191 on your system:
Most likely, nothing is listening. This port is empty.
Why Unassigned Ports Matter
Port 191 isn't technically unassigned—it has an official assignment to Prospero—but functionally it's unused, which raises an interesting question: should extinct protocols keep their port numbers forever?
The well-known ports range is finite. Only 1,024 numbers exist in this space. Many are allocated to protocols no one uses anymore. Port 191 is one of hundreds of reserved ports for dead services.
IANA could reclaim these numbers, but doing so creates risk. Someone might still be running an ancient Prospero server somewhere. Reassigning the port would break that system. So the port numbers stay reserved, monuments to protocols that didn't make it.
The Story of Prospero
B. Clifford Neuman designed Prospero while working on his Ph.D. at the University of Washington in the late 1980s and early 1990s. He published his dissertation research as "The Prospero File System: A Global File System Based on the Virtual System Model" in 1992.7
Neuman went on to become a principal designer of the Kerberos authentication system at MIT's Project Athena. Kerberos succeeded where Prospero didn't—it's still widely used today for network authentication. But both systems shared a similar philosophy: build tools that let users and systems create their own secure, customized environments.
Prospero was used to locate information from Internet archive sites in the early 1990s,8 but as the Web exploded in popularity, the need for a separate directory service disappeared. The protocol faded away, leaving only port 191 as evidence it existed.
What This Port Carries
Nothing. Port 191 carries the absence of what could have been—a more personal, more customizable Internet where you organized resources your way instead of searching through someone else's index.
Every time a search engine returns results ranked by an algorithm you don't control, remember port 191. Someone once built a system where you decided how to organize what you found. That system got a permanent port number. Then everyone chose the simpler option, and the port went dark.
Port 191 is a reminder that technical merit doesn't guarantee survival. Sometimes the more flexible, more powerful system loses to the one that's just easier to use.
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