Port 2578: RVS ISDN DCP — A Registered Fossil

What Port 2578 Is

Port 2578 sits in the registered ports range (1024–49151)—the middle tier of the port numbering system. Unlike well-known ports (0–1023), which are reserved for core Internet infrastructure, registered ports are claimed by applications and services through IANA. Anyone can register one. Many did, in the 1990s, for software that no longer runs anywhere.

Port 2578 is one of those.

IANA lists it as rvs-isdn-dcp, registered by Michael Zirpel of RVS Datentechnik GmbH in Munich. The service: RVS ISDN Device Control Protocol.

What RVS ISDN DCP Was

ISDN—Integrated Services Digital Network—was how Europe got fast(-ish) Internet and digital phone lines before broadband arrived. Germany adopted it aggressively. By the late 1990s, millions of German homes and offices had ISDN lines.

RVS Datentechnik built RVS-COM, communications software that let a single ISDN line serve an entire office. The idea: one machine connects to the ISDN hardware and acts as a server. Other machines on the LAN talk to it through DCP—the Device Control Protocol on port 2578. Through that server, they could send faxes (G3/G4), transfer files, check voicemail, and make calls. One ISDN adapter, many users.

Cisco 800-series routers supported this too—they could act as the DCP server on port 2578, letting RCAPI client software on Windows machines reach ISDN through the router over a standard LAN.¹

Why It No Longer Matters

ISDN is gone. Germany, ISDN's biggest market, completed its shutdown in 2019. The UK, France, Japan—all finished theirs around the same time. The hardware that DCP controlled no longer connects to anything.

RVS-COM hasn't been meaningfully developed in years. The Archive.org snapshot of RVS-COM Lite carries timestamps from the late 1990s and early 2000s.²

Port 2578 remains in the IANA registry because IANA doesn't un-register ports—they accumulate forever, like sediment. The number is taken. Nobody uses it.

If This Port Is Open on Your System

If you see port 2578 listening, one of three things is true:

1. Old software — Something from the RVS-COM era is still installed and running.
2. Coincidence — A completely unrelated application claimed this port dynamically.
3. Something worth investigating — Unknown open ports always deserve scrutiny.

To check what's actually using it:

macOS / Linux:

lsof -i :2578

Windows:

netstat -ano | findstr :2578
# Then look up the PID:
tasklist /FI "PID eq <PID>"

The Registered Ports Range

The registered range exists because the well-known ports (0–1023) ran out of room for legitimate services that didn't belong at the infrastructure level. Registered ports don't require special permissions to use—any process can bind to them—but IANA keeps a record of who claimed what, to reduce collisions.

In practice, many registered port assignments are just as dead as 2578. The registry is a historical record as much as an operational one. What it tells you is that someone, at some point, thought this service would matter enough to register a number for it.

For RVS ISDN DCP, they were right—for about a decade.