Port 187 sits in the well-known ports range with an official assignment, but if you go looking for what actually runs on it, you'll find almost nothing. It's a ghost port—officially claimed but functionally absent.
What Port 187 Is Assigned To
According to IANA's official registry, port 187 is assigned to aci (Application Communication Interface), available on both TCP and UDP.1 The contact listed is Rick Carlos. That's the extent of what the official record tells us.
The Problem: No Protocol Documentation
Here's what's strange: there's no RFC for ACI. No protocol specification. No implementation guide. No evidence that Application Communication Interface was ever deployed as a working protocol. Port 187 has a name in the registry, but the thing it's supposed to represent appears not to exist.
This happens more than you'd think. In the early days of the Internet, organizations and individuals could request port assignments for protocols they were developing. Some of those protocols made it into production. Others didn't. Port 187 appears to be one that didn't.
What Range This Port Belongs To
Port 187 is in the well-known ports range (0-1023). These are the ports assigned by IANA for system-level services that typically require privileged access to use. When a port in this range has an official assignment, that assignment is meant to be respected—even if the protocol is dormant.
In practice, because there's no active ACI protocol, port 187 sometimes gets used for other purposes: game servers, remote administration tools, or custom applications. But these are unofficial uses. Technically, the port is reserved.
Why This Matters
Ghost ports like 187 are a reminder that the Internet's infrastructure includes not just what's actively running, but also what was planned and never completed. The registry is a historical record as much as a technical directory.
Port 187 also highlights a reality of the well-known ports range: once assigned, a port stays assigned, even if the protocol dies or never launches. The namespace is permanent.
Security Considerations
Because port 187 has no standard protocol, anything listening on it is either:
- A custom application using the port unofficially
- Potentially malicious software (historically, some trojans have used unassigned or dormant ports)
If you see traffic on port 187, investigate what's actually running. There's no legitimate widespread service that should be using it.
How to Check What's Listening
On Linux/Mac:
On Windows:
If something is listening on port 187, it's worth understanding what that something is, because it's not the protocol the port was assigned for.
Related Ports
Other dormant or obscure well-known ports that were assigned but see little or no use include ports in the mid-100s range, many of which were reserved for early experimental protocols.
Frequently Asked Questions
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