Port 747 sits in the well-known range (0-1023), officially assigned by IANA to Fujitsu for device control services. But the reality on networks tells a different story.
The Official Assignment
According to IANA, port 747 is registered for "fujitsu-dev" (Fujitsu Device Control) on both TCP and UDP.1 Fujitsu filed the paperwork. They got the port. The registry says so.
What Actually Happened
Apple's Mac OS X used port 747 for NetInfo—their network information services system that handled user accounts, groups, and other directory services in older versions of OS X.2 Not port 748. Not some registered port in the thousands. Port 747. The one Fujitsu already had.
This wasn't malicious. Apple likely needed a port for RPC-based services, found 747 available in their testing environment, and shipped it. By the time anyone noticed the conflict, millions of Macs were using it.
The Well-Known Port Range
Ports 0-1023 are supposed to be controlled. IANA assigns them. You need to request them officially. The idea is that well-known services get well-known ports, so everyone knows where to find them.
Port 747 shows what happens when two legitimate services claim the same port. Fujitsu had the official assignment. Apple had the deployed usage. Both were technically correct. Neither could talk to each other.
Checking What's Listening
On macOS:
On Linux:
On Windows:
If you see something listening on port 747, check what it is. It could be legacy Mac OS X services. It could be Fujitsu device control software. It could be something else entirely that found an "available" port.
Security Considerations
Port 747 has been flagged in security databases as having been used by malware in the past.3 This doesn't mean the port itself is dangerous—it means attackers sometimes use officially assigned ports because they look legitimate.
If you're running Fujitsu device control software, port 747 should be open. If you're not, it shouldn't be. NetInfo is deprecated on modern macOS, so contemporary Mac systems shouldn't need it either.
Why This Port Matters
Port 747 is a reminder that the Internet's port registry is more guideline than law. IANA maintains the official list. Organizations request assignments. But when someone ships software that uses a port, that port is claimed—regardless of what the registry says.
The system works through a combination of official coordination and practical reality. Most of the time these align. Sometimes they don't. Port 747 is one of the times they didn't.
Related Ports
Other ports in the Fujitsu range include various device control and management services. NetInfo, meanwhile, has been replaced by Open Directory on modern macOS systems, which uses different ports entirely.
Frequently Asked Questions About Port 747
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