Port 1990 has no official IANA-assigned service. It sits in the registered port range (1024–49151) — the middle tier of the port numbering system, where applications and vendors claim ports for their own use, often without formal registration.
IANA lists it as unassigned. But that doesn't mean nothing runs here.
The Cisco STUN Connection
Cisco's Serial Tunnel protocol — abbreviated STUN, confusingly distinct from the NAT-traversal STUN — uses a block of four consecutive ports for priority queueing:
| Port | Priority |
|---|---|
| 1990 | Medium |
| 1991 | Normal (default) |
| 1992 | Low |
| 1994 | High |
STUN was Cisco's solution to a specific 1990s problem: IBM mainframes spoke SNA and SDLC — protocols that had nothing to do with IP — but companies were building IP networks everywhere and didn't want to run separate cables for their mainframe traffic. STUN tunneled those legacy serial frames across IP infrastructure, letting IBM and IP coexist on the same wire.1
Port 1990 carried the medium-priority traffic in that system. Whether the year 1990 and the port number matching is coincidence or a quiet engineer's joke is lost to time.
What Range This Port Belongs To
Ports 1024–49151 are the registered ports. Unlike well-known ports (0–1023), which require root/administrator privileges to bind and carry globally recognized services like HTTP and SSH, registered ports are open territory. Any application can use them. IANA maintains a registry of assignments, but registration is voluntary — vendors often just pick a number and ship.
This means port 1990 may be:
- Idle on most systems — no service has claimed it
- Occupied by Cisco STUN on legacy enterprise networks with IBM mainframe infrastructure
- Used by custom applications that picked this port for reasons of their own
Checking What's Listening on Port 1990
On Linux or macOS:
On Windows:
If nothing appears, nothing is listening. That's the most likely result on a modern machine.
Why Unassigned Ports Matter
The port namespace is a shared resource. When nothing is formally assigned to a port, it's available for anyone to use — which is both a feature (flexibility) and a problem (collision risk). Two applications on the same machine can't share the same port number for the same protocol.
Unassigned ports in the registered range aren't empty in any meaningful sense. They're just unclaimed at the registry level. In practice, they fill up constantly: development servers, game clients, enterprise software, legacy protocols. Port 1990 is a small example of this — formally unassigned, historically occupied, currently quiet on most systems.
Frequently Asked Questions
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