Port 2155: brdptc — The Door with No One Behind It

Port 2155 sits in the registered port range (1024–49151) and carries an official IANA assignment: a protocol named brdptc, short for "Bridge Protocol," registered on September 27, 2007, by a contact named Hideki Hatta. ¹

That is nearly everything publicly known about it.

No RFC. No open-source implementation. No vendor documentation. The name "Bridge Protocol" suggests something that relays or translates between network segments, but without documentation, that's speculation. Whatever problem brdptc was meant to solve, it never entered public circulation in any meaningful way.

What the Registered Range Means

Ports 1024–49151 are the registered ports — the middle tier of the port system. Unlike well-known ports (0–1023), which require root/administrator privileges to bind and carry protocols like HTTP, HTTPS, and SSH, registered ports can be used by ordinary applications. IANA maintains a registry for this range so that developers can request a stable, globally recognized home for their protocol.

The idea is coordination: if your application needs a consistent port number, you register it, and nobody else is supposed to use it for something else. In practice, the registry is voluntary, enforcement is nonexistent, and a port like 2155 can sit registered but practically dormant for nearly two decades.

What You Might Actually Find on Port 2155

If you discover traffic on port 2155 on a real system, it almost certainly isn't brdptc — it's whatever application decided to use that port for its own purposes. Some port-scanning databases have historically flagged 2155 as associated with older malware, though these associations are unverified and largely outdated. ²

The most honest answer is: if something is listening on port 2155 on your machine, it's using a registered-but-obscure port number because nothing more prominent owns it in practice.

How to Check What's Listening

To see what process has claimed port 2155:

On Linux or macOS:

sudo ss -tlnp | grep 2155
# or
sudo lsof -i :2155

On Windows:

netstat -ano | findstr :2155
# Then look up the PID:
tasklist | findstr <PID>

If nothing shows up, the port is quiet. If something does, the process name will tell you more than IANA ever could.

Why Ghost Assignments Matter

The port registry is a coordination system, not a surveillance system. Once a port is registered, IANA doesn't verify the protocol exists, ships, or is used. Ports can be registered and abandoned. Names can be claimed and never published.

Port 2155 is a small example of a larger truth: the registry reflects intent, not reality. The real question about any port isn't what IANA says lives there — it's what's actually listening.