Port 2781: whosells — A Registered Ghost

What This Port Is

Port 2781 sits in the registered ports range (1024–49151). Unlike well-known ports (0–1023), which are reserved for foundational Internet services, registered ports are claimed by specific applications through IANA upon request. IANA holds that registration indefinitely, even if the service disappears.

Port 2781 is registered for whosells, a service operated by ResolveNet IOM Ltd.

The whosells Service

ResolveNet IOM was a company that built a B2B procurement directory in the early 2000s. The idea: businesses could publish what they sell, and buyers could query a central directory to find suppliers. The whosells server was an HTTP-based query/response system — you asked "who sells X?" and it returned registered vendors.

The server ran at whosells.resolvenet.net. That hostname no longer resolves. ResolveNet IOM Ltd no longer appears to exist in any active capacity. The service is gone.

What remains is the IANA registry entry: port 2781, TCP and UDP, registered to a contact email at royere.net.¹ The port is spoken for, on paper, forever.

Security Notes

Some port databases flag 2781 as having been used by malware. This is worth understanding correctly: malware authors don't register ports with IANA. They pick ports opportunistically, and any unmonitored registered port is fair game. Port 2781 isn't inherently dangerous. Nothing listens on it by default on any modern system.

If you see unexpected traffic on 2781, investigate it — but don't assume the whosells registration explains it. Nobody is running whosells anymore.

Checking What's Listening on This Port

To see if anything is using port 2781 on your system:

macOS / Linux:

sudo lsof -i :2781

Windows:

netstat -ano | findstr :2781

If nothing is returned, nothing is listening. That's the expected result.

To check if port 2781 is reachable on a remote host:

nc -zv <host> 2781

Why These Ports Matter

The registered port range exists so applications can claim a consistent, predictable port for their service. The system works when services are healthy and actively used. When companies fold and services disappear, their port assignments become dead entries — reserved space in a namespace that nobody can reclaim easily.

Port 2781 is one of hundreds in this situation: formally assigned, practically unused, held by an organization that no longer exists. The IANA registry is, in part, a graveyard for the ambitions of the early Internet.