What This Port Is
Port 3004 sits in the registered port range (1024-49151), the middle tier of the port numbering system. Ports in this range can be registered with IANA for specific services, but they don't require root or administrator privileges to open. Any application on your system can bind to port 3004 without special permissions.
IANA has no official assignment here.1 The port is blank in the registry.
The One Notable Unofficial Use
Apple's iSync — a synchronization tool that kept iCal calendars and Address Book contacts in sync with mobile phones over Bluetooth and USB — used port 3004 for its communication.2
iSync was removed in OS X Mountain Lion (2012) and has not shipped since. If you're seeing activity on port 3004 today, it isn't iSync.
What Might Actually Be on Port 3004
Developers frequently use ports in the 3000s for local development servers. If you see port 3004 active on your machine, it's likely one of:
- A local development server (web framework, API server, microservice)
- Custom application or internal tool using a non-conflicting port
- A proxy or middleware component in a multi-service local stack
How to Check What's Listening
macOS / Linux:
or
Windows:
The output will show you the process ID (PID) and process name bound to the port.
Why Unassigned Ports Matter
The registered range exists as a commons — space for services to claim a stable home so clients know where to find them. HTTP picked 80. HTTPS picked 443. SSH picked 22. The registry is how those agreements become durable.
Port 3004 never got claimed. That's not unusual — most of the 48,127 ports in the registered range are empty. They're a reminder that the port space is vast and the Internet's protocols occupy only a fraction of it. The rest is available, waiting, or quietly repurposed by developers who just needed a number that wasn't taken.
Frequently Asked Questions
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