203

Port 203/tcp is registered with IANA under the service name at-3, description "AppleTalk Unused," and is dual-registered on 203/udp as well [Confirmed] (IANA Service Name and Transport Protocol Port Number Registry). The Assignee, Contact, Registration Date, Modification Date, and Reference/RFC columns are all blank in the registry for this entry — no RFC defines it, and no organization is credited as assignee, so those fields are recorded as blank/Unknown rather than invented. The port sits inside IANA's contiguous AppleTalk reservation spanning roughly 200–208 (e.g. 201=at-rtmp, 202=at-nbp, 204=at-echo, 208=at-zis), but IANA itself labels 203 specifically as unused — it was reserved for the AppleTalk suite but never assigned to a named sub-protocol. AppleTalk was Apple's proprietary local-area networking stack, largely retired after Mac OS X's mid-2000s shift to AppleTalk-over-IP and eventual removal, so this whole port block has been functionally dormant for roughly two decades. No source consulted in this pass documents any currently maintained software binding to 203/tcp, nor any malware/trojan association, nor internet-wide scanning-prevalence data — those aspects are honestly reported as Unknown below rather than asserted. A secondary port-lookup site's loose claim tying the 200s block to AppleTalk Zone Information Protocol (ZIP) could not be independently confirmed (source returned HTTP 403 on direct fetch) and is flagged accordingly.

IANA assignment

at-3 — "AppleTalk Unused"; reference (blank — no RFC cited); assignee (blank); dual-registered 203/tcp + 203/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?page=5

Range class

well-known (0–1023)

Prevalence

no scan/open-frequency data source was available in this pass (nmap-services frequency not consulted for this entry) [Unknown]

Related ports

the AppleTalk 200–208 reservation cluster (201=at-rtmp, 202=at-nbp, 204=at-echo, 208=at-zis) [Confirmed] — IANA registry

Primary use

reserved slot within the AppleTalk protocol suite's 200–208 port block; IANA marks it explicitly unused, not assigned to any named AppleTalk sub-protocol

Other/unofficial uses

none documented; one secondary source loosely associates the 200s AppleTalk block with Zone Information Protocol (ZIP), but this could not be independently verified (source fetch returned HTTP 403; only a search snippet was available)

Security implications

no malware/trojan currently associated with 203/tcp or 203/udp per legacy port-database checks; no internet-wide scanning/honeypot telemetry available (Shodan InternetDB excluded from this site per licensing)

Typically seen on

no current production use documented; AppleTalk itself is obsolete (superseded by TCP/IP, effectively retired since the mid-2000s Mac OS X transition)

Analyst note

an open 203/tcp is not expected on any modern host; treat as anomalous given AppleTalk's near-total obsolescence, though no specific threat signature is documented.

Port 203/udp is registered with IANA under the service name at-3, carrying the descriptive label "AppleTalk Unused." It sits inside a contiguous block of AppleTalk-suite port assignments — 201 (at-rtmp, Routing Table Maintenance Protocol), 202 (at-nbp, Name Binding Protocol), 203 (at-3), and 204 (at-echo, AppleTalk Echo Protocol) — reserved during the era when Apple's proprietary LAN networking stack needed IANA-registered slots on TCP/IP-capable hosts. Unlike its neighbors, though, IANA's own registry text marks 203 as "Unused": no protocol document, RFC, or formal specification was ever attached to this particular number, so it functioned as a reserved-but-vacant placeholder rather than an active AppleTalk service. The entry is dual-registered — 203/tcp carries the identical service name and description — and both the assignee and reference columns are blank in the IANA source data, a routine gap for older reservations rather than a data-quality problem. AppleTalk itself has been defunct for well over a decade: Apple dropped AppleTalk support from Mac OS X starting with 10.6 "Snow Leopard" (August 2009), and Linux more recently removed its legacy AppleTalk kernel support. No current mainstream software, daemon, or device documents legitimate use of UDP 203, and a port-database aggregator (auditmypc.com) records no virus/trojan association for it. No IANA assignment date is published for this entry, and the only exposure figure available is the nmap-services observed open-frequency of roughly 0.000461 for 203/udp against 0.000000 for 203/tcp, so an analyst encountering unsolicited UDP 203 traffic today should treat it as scanner noise, misconfiguration, or an unrelated application reusing the number rather than genuine AppleTalk activity.

IANA assignment

at-3 — "AppleTalk Unused"; reference (blank — no RFC cited in IANA registry); assignee (blank); dual-registered 203/tcp + 203/udp, identical name/description [Confirmed] — cached IANA CSV (the IANA Service Name and Transport Protocol Port Number Registry, lines 492–493) corroborated by iana.org registry text/xhtml

Range class

well-known (0–1023) [Confirmed] — port number range

Prevalence

nmap-services observed open-frequency 203/udp ≈ 0.000461 (very low — roughly 5 in 10,000 scanned hosts in the nmap-services sample) [Confirmed] — nmap-services dataset; the dual-registered 203/tcp row records 0.000000 in the same dataset [Confirmed] — nmap-services dataset. Shodan's published port-coverage list carries no entry for this port

[Unknown] — https://data-status.shodan.io/ports.html (absence, not a measurement)

Related ports

201/at-rtmp, 202/at-nbp, 204/at-echo (same AppleTalk assignment block) [Confirmed] — https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.txt

Registration date

not published — IANA's registry carries no per-entry assignment date for this entry; none inferred or invented [Confirmed] — https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.txt

Primary use

reserved-but-unused slot within the AppleTalk protocol suite's port block (201 at-rtmp, 202 at-nbp, 203 at-3, 204 at-echo); IANA's own label indicates no protocol was ever formally specified for this number

Other/unofficial uses

none documented

Security implications

no malware/trojan association on record (auditmypc.com explicitly lists "Virus/Trojan: No"); AppleTalk is obsolete (dropped from Mac OS X 10.6 "Snow Leopard," Aug 2009; Linux later removed legacy AppleTalk kernel support), so unsolicited traffic is more plausibly scanner noise or misconfiguration than genuine AppleTalk activity [Likely] — https://www.auditmypc.com/udp-port-203.asp, https://thisdayintechhistory.com/08/28/snow-leopard-released-the-end-of-appletalk/, https://www.privacyguides.org/news/2026/06/18/linux-removes-support-for-legacy-appletalk-protocol-in-response-to-ai-patches/

Typically seen on

historically, AppleTalk-era Mac/LocalTalk-bridge hosts reserving the block; in practice this specific number was never activated, so it is rarely if ever seen as legitimate traffic today