Port 1992: Cisco STUN Priority 3 / IPsendmsg — A quiet port with two old names

Port 1992 sits in the registered ports range (1024–49151), which means IANA has an entry for it. Two entries, actually. Neither one is something you're likely to encounter in the wild today.

What the Registered Range Means

Registered ports are the middle tier of the port numbering system. Below them, the well-known ports (0–1023) carry the Internet's core protocols — HTTP, SSH, DNS. Above them, the dynamic/ephemeral ports (49152–65535) are handed out temporarily for outbound connections.

Registered ports are different. Anyone can apply to IANA to claim one, and IANA will record the association. But registration isn't enforcement. Nothing stops you from running any service on any registered port. The registry is a coordination mechanism, not a lock. Port 1992 was registered twice, by two different groups, for two different purposes.

Cisco STUN — Serial Tunnel Priority 3

Cisco's Serial Tunnel protocol (STUN) was a way to carry SDLC (Synchronous Data Link Control) traffic across IP networks. SDLC was IBM's protocol for connecting terminals to mainframes in SNA (Systems Network Architecture) environments. Before STUN, organizations running SNA needed dedicated leased lines everywhere. STUN let them reuse their existing IP infrastructure instead.

To support traffic prioritization, Cisco carved up four port numbers for STUN peer connections, one per priority level:

• Port 1990 — Priority 1 (highest)
• Port 1991 — Priority 2
• Port 1992 — Priority 3
• Port 1994 — Priority 4 (lowest)

The idea was that time-sensitive traffic could flow on higher-priority connections while bulk transfers used lower ones. Port 1992 was the third tier of that queue. STUN itself is a dead technology — SNA environments have been decommissioned for decades — so this port assignment is effectively a historical footnote. ¹

IPsendmsg — A Note from Sydney

The second IANA registration for port 1992 is ipsendmsg, listed as "IPsendmsg" with a contact from the University of Sydney: Bob Kummerfeld, who was involved in building early networking infrastructure in Australia, including the SUN (Sydney University Network) protocol suite in the 1980s.

The IANA registry entry includes a candid note: "This entry records an unassigned but widespread use."

That's the registry admitting the port was already in use before anyone filed the paperwork. Whatever IPsendmsg was — a messaging facility built on top of IP, likely for inter-process or inter-host communication — it was deployed and running at enough sites that IANA formalized the entry after the fact. ²

This is more common in networking history than you might expect. Protocols got built, deployed, and spread before the standards organizations caught up.

What's Actually Listening on Port 1992 Today

Almost certainly nothing, unless you've specifically configured something to use it. To check on your own system:

Linux / macOS:

ss -tlnp | grep 1992
# or
lsof -i :1992

Windows:

netstat -ano | findstr :1992

If something shows up, cross-reference the process ID against your running applications. Port 1992 doesn't carry any well-known active service, so an unexpected listener here is worth investigating.

Why Unassigned (and Lightly Assigned) Ports Matter

Port 1992 illustrates something real about the port registry: it's a record of history as much as it is a technical specification. The STUN priority ports were registered when SNA was a going concern. IPsendmsg was registered to acknowledge something that had already happened.

The registry exists so that new software doesn't accidentally collide with established uses. When you pick a port for your application, you check here first. The fact that port 1992 shows two old registrations is useful information: don't use it without checking what you might be shadowing, even if the original tenants moved out decades ago.