What Port 2561 Is
Port 2561 is registered with IANA under the name MosaixCC, assigned to both TCP and UDP. It sits in the registered port range — the middle tier of the port number space, between 1024 and 49151, where companies and developers formally claim ports for specific applications.
No active service uses this port today. It is an orphan port: officially claimed, historically real, and functionally abandoned.
The Registered Port Range
Ports 1024 through 49151 are called registered ports. Unlike the well-known ports below 1024 (which are tightly controlled and reserved for foundational protocols like HTTP, SSH, and DNS), registered ports are available to any organization willing to file a request with IANA.
The system made sense in the 1990s, when software vendors needed stable port numbers so their products could reliably communicate across networks and firewalls. A company would register a port, ship software that used it, and the port number would be documented for network administrators who needed to open or monitor it.
The problem: companies get acquired, go out of business, or simply abandon old products. The registry doesn't expire. Port 2561 is one of hundreds of registered ports that now point to nothing.
Who Was Mosaix?
Mosaix, Inc. was headquartered in Redmond, Washington, and made predictive dialing and call center management software in the 1990s. Their product line included blended inbound/outbound dialing platforms, campaign management tools, and workforce optimization software.
Around 1999, Mosaix was acquired by Lucent Technologies. The software and company name were absorbed into Lucent's portfolio and eventually disappeared through further corporate restructuring. The "CC" in MosaixCC almost certainly stood for Call Center.
The port registration remains in IANA's records — a small fossil of a company that built real infrastructure for real call centers, then got swallowed by a larger world.
Is Anything Running on This Port?
Almost certainly not anything intentional. SANS Internet Storm Center occasionally logs scanning activity against port 2561, but this reflects automated Internet-wide scanning rather than active service traffic — bots probe every port looking for anything that responds.1
If you see traffic on port 2561 on your own systems, it warrants investigation. The most likely explanations are:
- Automated port scanning from external sources (normal background noise)
- A misconfigured application that happened to bind to this port
- Something you didn't install intentionally
How to Check What's Listening on This Port
On Linux/macOS:
On Windows:
If nothing returns, nothing is listening. If something does return, the process name or PID will tell you what it is.
Why Orphan Ports Matter
The registered port range is full of ports like 2561 — legitimately assigned to real software from real companies that no longer exist. This creates a few practical effects:
For firewall rules: You may have inherited firewall configurations that reference port 2561 without knowing why. If no one can explain the rule, it probably doesn't need to exist.
For security scanning: Orphan ports occasionally get co-opted. If malware or an unauthorized service wants to listen on a port that looks "official" and generates no alerts, a forgotten registered port is a reasonable choice. Seeing unexpected traffic on port 2561 is more suspicious, not less, precisely because there's no legitimate software that should be using it.
For the registry itself: IANA's port registry is a historical document as much as an operational one. Many registrations date from an era when the Internet was smaller and corporate continuity was assumed. The registry reflects the Internet's past as faithfully as its present.
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