191

Port 191 is registered with IANA on both TCP and UDP under the service name prospero, described as "Prospero Directory Service," with B. Clifford Neuman listed as both assignee and contact and no RFC cited in the registry's Reference column. Prospero itself was an object-oriented distributed directory and file-location protocol that Neuman designed at the University of Washington and later USC/Information Sciences Institute in the early 1990s, formally documented together with Susan Augart in a June 1993 specification. A Prospero client resolved object references — called Prospero URLs — by querying a directory server listening on this port, which returned access-method pointers so the underlying data could actually be fetched over whichever transport hosted it; contemporary write-ups cite FTP, NFS, AFS, WAIS, and Gopher as the backing transports Prospero could point to. A related but distinct prospero URI scheme was later published as RFC 4157 in June 2005; that document standardizes URI syntax only, not the port-191 service assignment, so it should not be read into the IANA Reference field. Independent sources describing Prospero's history consistently note the protocol saw little production deployment and was effectively abandoned by the early 2000s; no actively maintained client or server implementation, CVE, or exposure/scanning writeup naming port 191 was located in this pass, so any exposure claim beyond "the protocol is essentially disused today" would be speculation rather than a sourced finding.

IANA assignment

prospero — "Prospero Directory Service"; Reference column blank (no RFC cited); assignee/contact B. Clifford Neuman; dual-registered 191/tcp + 191/udp [Confirmed] — https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml (cross-checked against cached registry CSV, lines 468–469)

Range class

well-known (0–1023)

Registration date / modification date / service code / unauthorized-use-reported / assignment notes

all blank in the IANA registry (both tcp and udp rows) — left Unknown, no date or reference fabricated [Unknown] — cached registry CSV lines 468–469

Primary use

object-oriented distributed directory/naming and file-location protocol designed by B. Clifford Neuman (Univ. of Washington, later USC/ISI); formally specified with Susan Augart in June 1993; clients resolved "Prospero URLs" via a directory server returning access-method pointers into other transports (FTP/NFS/AFS/WAIS/Gopher) [Likely] — https://www.usenix.org/legacy/publications/compsystems/1992/fall_neuman.pdf, https://www.w3.org/Addressing/URL/4_1_Prospero.html

Other/unofficial uses

none identified beyond the historical Prospero implementations themselves [Unknown]

Security implications

no CVE, exploit-DB entry, or scanning-campaign writeup found for port 191 or Prospero in this pass; given the protocol's documented obsolescence, live exposure is expected to be negligible, but that is an inference from historical sourcing, not a measured scan statistic [Unknown]

Typically seen on

no modern hosts identified; historically USC/ISI-era Prospero directory/file-location servers (early-to-mid 1990s) [Likely]

Related but distinct standard

RFC 4157 (June 2005, Informational) documents a "prospero" URI scheme — this concerns URI syntax, not the IANA port-191 reference, and must not be recorded as the port's IANA Reference [Confirmed] — https://datatracker.ietf.org/doc/html/rfc4157

Current deployment status

no actively maintained implementation found; sources describe the protocol as largely unimplemented/abandoned by the early 2000s [Likely] — https://www.w3.org/wiki/UriSchemes/prospero

Port 191/udp is registered with IANA under the service name prospero, described as "Prospero Directory Service," with assignee B. Clifford Neuman and a blank reference field. The registration is a dual assignment: the identical service name, description, and assignee also appear for 191/tcp in the same registry, so the two transports share one entry rather than having independently-assigned meanings. Prospero was a distributed directory and filesystem protocol developed at USC's Information Sciences Institute by B. Clifford Neuman in the early 1990s. It let users browse and index files scattered across many anonymous-FTP archives through a unified virtual directory, functioning as a conceptual precursor to later web search and file-indexing tools before HTTP-based search engines took over that role in the mid-to-late 1990s. No actively maintained modern client or server software advertises this port today; Prospero is treated as an obsolete, historical protocol in secondary references rather than a live service family. No CVEs, documented exploits, or notable scanning/abuse campaigns targeting 191/udp were found during this research pass; the port appears only in generic legacy-port reference lists (e.g., OSCP-style cheat sheets, ISC SANS port lists), not as an active attack surface, though the absence of reports is not the same as a confirmed all-clear.

IANA assignment

prospero — "Prospero Directory Service"; reference blank (no RFC cited); assignee B. Clifford Neuman; dual-registered 191/tcp + 191/udp [Confirmed] — IANA Service Name and Transport Protocol Port Number Registry (https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml)

Range class

well-known (0–1023) [Confirmed]

Registration/modification date

Unknown — blank in the cached IANA registry CSV; not fabricated [Confirmed absence]

Related ports

191/tcp (identical dual registration); no other closely related well-known ports identified

Primary use

Prospero Directory Service — distributed directory/filesystem indexing protocol from USC/ISI (B. Clifford Neuman), used in the early 1990s to index and browse files across anonymous-FTP archives; a conceptual precursor to web search engines

Security implications

no CVEs, documented exploits, or notable scanning/abuse campaigns identified for 191/udp; appears only in generic legacy/rarely-seen port reference lists; UDP's spoofability is a generic theoretical amplification/DoS concern for any exposed legacy UDP service, not something specifically reported here [Unknown] — https://isc.sans.edu/services.html, https://github.com/rewardone/OSCPRepo/blob/master/CheetSheets/Networking/ServicesPortsList.txt

Typically seen on

essentially never seen in modern scans; a response would be anomalous/legacy rather than expected

Common/current software

no actively maintained modern client or server implements this protocol; superseded by web-based search by the mid-to-late 1990s [Likely] — http://www.t1shopper.com/tools/port-number/191/

Analyst note

an open/responsive 191/udp is very unusual today; treat as a legacy artifact, misconfiguration, or possible decoy rather than an expected service, given the near-total absence of modern deployment or abuse reporting.