Port 3681: BTS X73 — A Medical Protocol That Never Arrived

What This Port Is

Port 3681 sits in the registered port range (1024–49151). These ports are assigned by IANA to specific services upon request — unlike the well-known ports below 1024 (which carry the foundational protocols of the Internet) or the ephemeral ports above 49151 (which operating systems use temporarily for outgoing connections).

IANA records port 3681 as assigned to BTS X73 — service name bts-x73 — registered in January 2003.¹

What BTS X73 Was

X73 refers to the ISO/IEEE 11073 family of standards for medical device communication — the protocols that define how patient monitors, blood glucose meters, pulse oximeters, and similar devices exchange data.²

BTS X73 was apparently a Bluetooth transport binding for this protocol: a way to carry X73 medical device data over Bluetooth connections. In 2003, the standard was new, Bluetooth was young, and someone registered a port anticipating a network future that didn't quite arrive in the way they imagined.

The ISO/IEEE 11073 standards did evolve and continue to be used in healthcare interoperability — but this particular port assignment left no footprint. No RFCs reference it. No major software packages use it. No firewall documentation flags it. The registration exists; the usage doesn't.

What You'll Find on This Port

Almost certainly nothing. Port 3681 does not appear in observed traffic data, security advisories, or software documentation in any meaningful way.

If you're seeing traffic on port 3681 on your network, it is more likely:

• A custom application that happened to choose this port
• Malware or scanning tools using it opportunistically
• A misconfigured service intended for a different port

It is not coming from an active BTS X73 deployment.

How to Check What's Listening

To see if anything is bound to port 3681 on your system:

Linux / macOS:

# Show processes listening on port 3681
ss -tlnp sport = :3681

# Or with lsof
lsof -i :3681

Windows:

netstat -ano | findstr :3681

If these return nothing, the port is closed — which is the expected result on any normal system.

Why Registered Ports Like This Matter

The registered port range is vast: over 48,000 possible ports. IANA manages assignments to prevent conflicts — if two popular services both tried to use port 3681, network administrators couldn't reliably configure firewalls or diagnose traffic.

But the registry also accumulates history. Services registered in the early 2000s for technologies that never gained adoption sit alongside ports for software used by millions. Port 3681 is in good company: thousands of registered ports are effectively dormant, artifacts of protocols that seemed promising at the moment of registration.

The system works not because every registered port is active, but because the registration itself reserves the number. If BTS X73 were to return in some future form, the port would still be waiting.