Port 3718: OPUS Server Port — Registered, Then Forgotten

What Port 3718 Is

Port 3718 sits in the registered ports range — the stretch of port numbers from 1024 to 49151 that IANA administers for specific applications and services. Unlike the well-known ports below 1024 (where HTTP, SMTP, SSH, and their peers live), registered ports don't require elevated system privileges to open. Any application can bind to one.

IANA's registry lists port 3718 as assigned to "OPUS Server Port", service name opus-services, for both TCP and UDP. The registration dates to March 2003.¹

That's essentially where the trail ends.

The OPUS Registration

No public RFC documents OPUS. No open-source project claims it. No vendor documentation references port 3718 as a standard. The registration was made by an individual named Detlef Stoever, and whatever OPUS Server was intended to be — a private enterprise application, an internal tool, a project that never shipped — it left no public footprint.²

This is more common than you'd expect. The IANA registry contains hundreds of ports registered in the early-to-mid 2000s by individuals and small companies for applications that were never widely deployed, were abandoned, or simply stayed internal. The registry is a record of intention, not of adoption.

Port 3718 is officially claimed. It is practically unclaimed.

What You Might Actually Find on Port 3718

Because the registered assignment is obscure, port 3718 behaves like an unassigned port in practice. That means:

• Development servers — Developers routinely pick arbitrary high-numbered ports for local services to avoid conflicts with well-known assignments.
• Custom application traffic — Internal enterprise software sometimes picks ports from this range at random.
• Port scanners and security tools — These will flag an open 3718 as unusual precisely because there's no recognized service to explain it.

If you see port 3718 open on a machine you manage, that's worth investigating. There's no standard answer for what it should be.

How to Check What's Listening

On Linux or macOS:

# Show what process has port 3718 open
ss -tlnp | grep 3718
# or
lsof -i :3718

On Windows:

netstat -ano | findstr :3718

Then cross-reference the process ID against your running processes. If you don't recognize what's there, that's your next question to answer.

Why These Ports Matter

The registered port range exists so that applications have stable, predictable addresses to advertise to their users. When a service claims a port with IANA, it's saying: "this is our door, and the world should know it." The system works when registrations correspond to real, identifiable software.

Port 3718 illustrates the limit of that system. A registration was made. The software either never launched publicly, stayed private, or faded away. The claim persists in the registry while the thing that made the claim has gone quiet.

The port sits open in the middle of the registered range — not quite unassigned, not quite used. A small administrative ghost.