What This Port Does
Port 3106 is officially registered with IANA as cardbox-http — the HTTP interface for the Cardbox Licence Server.
Cardbox is a Windows database program with an unbroken lineage back to 1982. It lets organisations build and search structured databases: contact records, museum catalogues, document archives, medical records. The software runs on a client-server model: the main Cardbox Server listens on port 3105, and when you also run the Cardbox Licence Server on the same machine, it needs its own port. That port is 3106.1
The separation matters for a practical reason: two processes on the same host cannot share a port. So Cardbox reserves the adjacent number. Port 3105 handles database queries. Port 3106 handles licence administration — verifying that clients are authorised to connect, and managing licence borrowing for users who work offline.2
The Range It Belongs To
Port 3106 sits in the registered ports range (1024–49151). These ports are assigned by IANA to specific applications and services. Unlike the well-known ports below 1024 — which require root or administrator privileges to bind — registered ports can be used by any process. Any program can technically open port 3106, but IANA's registry records it as reserved for Cardbox HTTP.3
An Honest Assessment of Obscurity
Cardbox is genuinely rare software. It has a small, loyal user base — libraries, law firms, specialist archives — and has been maintained by the same small team since 1982. That longevity is remarkable. That most people have never heard of it is also true.
Port 3106 will appear on almost no networks. If you see traffic on this port, you are almost certainly running Cardbox, or something is scanning for open ports and happened to knock on this door.
How to Check What Is Listening Here
If you want to know whether anything is using port 3106 on your system:
On Linux or macOS:
On Windows:
If nothing responds, the port is closed — which is the expected state on nearly every machine in the world.
Why Unassigned-Feeling Ports Still Matter
Port 3106 is registered, not unassigned — but it functions like an unassigned port in practice because Cardbox is so rarely deployed. This is common across the registered range: thousands of ports are officially spoken for by software that runs on a handful of servers worldwide.
The registry isn't just a list of what's common. It's a record of what was claimed — a way to prevent collisions between applications that might otherwise independently choose the same port. Port 3106 being reserved for Cardbox means no other application should register a competing service there, even if Cardbox itself is running nowhere near you.
Frequently Asked Questions
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