Port 2037: APplus Application Server — A Name Tag That Not Everyone Reads

What This Port Is

Port 2037 is a registered port, officially assigned by IANA to the APplus Application Server — the communication endpoint for APplus ERP, a browser-based enterprise resource planning system built by Asseco Solutions for small and mid-sized manufacturers.¹

The registration has been on the books since June 2008, updated as recently as July 2021, covering both TCP and UDP. The original registration listed it as "P2plus Application Server" before being renamed to APplus.²

What APplus Actually Is

APplus is a German-made ERP platform used by over 1,800 companies, primarily in manufacturing. It handles production planning, inventory, CRM, finance, and project management through a web browser. Its application server component — the piece that actually runs the backend logic and serves the browser-based interface — communicates on port 2037.³

This is the kind of port that matters deeply to the businesses running APplus, and is essentially invisible to everyone else.

The "Unauthorized Use Known" Flag

Here's the interesting part. The IANA registry entry for port 2037 includes a notation that does not appear on most port assignments: "Unauthorized Use Known."²

IANA uses this annotation to flag ports where traffic has been observed that does not belong to the registered service. It does not specify what that traffic is — only that the port is not exclusively used by APplus in practice.

This happens in the registered port range more often than people expect. A developer picks an available port for an internal tool, a piece of malware picks one at random, or a service grows popular before anyone thinks to register it officially. The result is a port with a legal owner and an unknown squatter.

What Range This Port Belongs To

Port 2037 sits in the registered port range: 1024 through 49151.

Unlike the well-known ports (0–1023), which require elevated privileges to bind and are tightly controlled, registered ports are more loosely governed. Any application can use them without special system permissions. IANA maintains a registry of assignments, but enforcement is practical rather than technical — nothing stops software from binding to a registered port that officially belongs to someone else.

This is precisely how "Unauthorized Use Known" becomes a real annotation in an official registry.

How to Check What Is Listening on Port 2037

If you see traffic on port 2037 and need to identify the process behind it:

Linux / macOS:

# Show what process is bound to port 2037
sudo ss -tlnp | grep 2037

# Or with lsof
sudo lsof -i :2037

Windows:

:: Show connections and listening ports with process IDs
netstat -ano | findstr :2037

:: Then look up the process by PID
tasklist | findstr <PID>

If you are running APplus, traffic on this port is expected. If you are not running APplus and see activity here, it warrants investigation.

Why Unassigned (and Lightly-Used) Ports Matter

Most of the 65,535 ports are quiet most of the time. The registered range in particular is a vast territory where official assignments sit next to undocumented uses, forgotten services, and occasional unauthorized occupants.

This is not a flaw — it is how the system scales. Rather than a locked registry where every port is claimed and enforced, the Internet uses a looser convention: register if you want to, check if you need to, investigate when something unexpected appears.

Port 2037 is a small example of that complexity. It has a name, an owner, a registration date — and a footnote acknowledging that reality does not always match the paperwork.