6

Port 6/tcp carries no IANA service assignment: it is listed as "Unassigned" in the IANA Service Name and Transport Protocol Port Number Registry, with no service name, no assignee, no registration or modification date, and a blank Reference field. That blank-everything state is itself the fact worth recording — port 6 has been unassigned since the earliest port lists, appearing as "Unassigned" already in RFC 1340 (the 1992 Assigned Numbers RFC) and remaining unassigned in the live registry as of June 2026. The companion 6/udp row is identically Unassigned, so there is no transport-specific carve-out. For an analyst the practical point is that no standard software binds to port 6/tcp by default, so any process listening on it is non-standard and any inbound connection attempt is anomalous. A common trap in third-party port databases is to bleed the nearby compressnet service name (which IANA assigned to ports 2 and 3, not 6) onto port 6 — that attribution is wrong, and the ports 2/3 compressnet assignments were themselves de-assigned in early 2025, so neither the name nor a neighbor's assignment belongs here. Threat-wise there is no CVE, exploit, or named malware family documented as targeting port 6/tcp specifically; SANS ISC honeypot data shows only low-volume, distributed daily probe traffic with no service banners or exploit payloads — the background-radiation pattern of generic low-port sweeps rather than targeted exploitation. The standard posture is firewall block-and-log, treating any hit as reconnaissance noise, misrouted traffic, or a misconfigured application worth a second look rather than a known service.

IANA assignment

Unassigned — no service name, no assignee, blank Reference field; 6/udp identically Unassigned (no transport-specific assignment) [Confirmed] — IANA Service Name and Transport Protocol Port Number Registry (the IANA Service Name and Transport Protocol Port Number Registry, line 14)

Range class

well-known (0–1023)

Registration / modification date

blank in the registry (genuinely empty, not withheld) [Confirmed] — IANA registry

IANA Reference

blank — no RFC cited [Confirmed] — IANA registry

Related ports

the low well-known cluster around it — compressnet at 2/3/tcp (the misattribution source), echo 7/tcp

Primary use

none — no assigned protocol or standard service; listed "Unassigned" as far back as RFC 1340 (1992) and still Unassigned in the live registry as of June 2026

Other/unofficial uses

none legitimate; the compressnet name is sometimes misattributed to port 6 in third-party databases — IANA assigned compressnet to ports 2 and 3 (not 6), and those 2/3 assignments were de-assigned in early 2025

Common software

none binds to 6/tcp by default; any listener is non-standard

Security implications

no CVE, exploit, or named malware family documented for this port; SANS ISC records low-volume distributed daily probes (~20+ unique sources in a single-day snapshot, 3–13 records each) with no banners or payloads — background low-port sweep noise

Typically seen on

nothing standard — an open or probed port 6 is anomalous; treat inbound as recon noise, misrouted traffic, or a misconfigured app

Analyst note

Because the port is unassigned with no legitimate service, any inbound connection is anomalous. Firewall block-and-log is the standard posture; don't let third-party "compressnet on port 6" labels mislead triage.

Port 6/udp is listed as Unassigned in the IANA Service Name and Transport Protocol Port Number Registry: it carries no service name, no description, no assignee, no registration or modification date, and a blank reference field. It sits in the well-known range (0–1023) but, unlike its TCP counterpart, has never been allocated. The TCP side — 6/tcp — is itself unassigned in the modern IANA registry; the number 6 is more often remembered in the context of TCP Port Service Multiplexer (TCPMUX, RFC 1078), but that history attaches to the protocol-number 6 / port-1 lineage rather than to a UDP assignment on port 6, and no part of it extends to UDP here. Because the slot is unassigned and tied to no standard service, the practical analyst takeaway is simple: there is no legitimate, documented application that should be speaking UDP on port 6. Any observed traffic is therefore anomalous by default and is commonly treated by perimeter devices as something to drop or investigate — a probe artifact, a misconfiguration, a scanner sweeping the low ports, or a non-standard channel chosen precisely because it is unremarkable. No CVEs, malware families, or exploitation campaigns specifically targeting port 6/udp as a service were found in publicly available sources as of 2025–2026; the security posture here is the generic one for an unassigned well-known port, not a port-6-specific incident history. Treat a responsive 6/udp as a finding to chase down rather than a service to identify.

IANA assignment

Unassigned — no service name, no description, no assignee; reference field blank (no RFC cited) [Confirmed] — IANA Service Name and Transport Protocol Port Number Registry (the IANA Service Name and Transport Protocol Port Number Registry, line 15)

Range class

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

Registration / modification dates

none recorded (Unassigned slot) [Confirmed] — IANA registry; not fabricated

Related ports

6/tcp (also Unassigned); the low well-known band scanners commonly sweep

Primary use

none — no IANA-assigned use on UDP; the port-6 / TCPMUX history (RFC 1078) attaches to TCP/protocol-number lineage and does not extend to 6/udp

Common software

none documented; no software legitimately uses 6/udp

Security implications

unassigned → any traffic is anomalous by default; perimeter devices routinely drop/flag it (possible probe, scanner sweep, misconfiguration, or non-standard channel). No port-6-specific CVEs or campaigns found 2025–2026

Typically seen on

nothing standard — a responsive 6/udp is an anomaly to investigate

TCP/UDP dual status

both 6/tcp and 6/udp are Unassigned in the modern registry (shared Unassigned status, not a shared assignment) [Confirmed] — IANA registry (lines 14–15)

Analyst note

No legitimate service expected on 6/udp; treat any response as a finding to chase, not a service to fingerprint.