Port 737 belongs to the well-known port range (0-1023)—the ports assigned by the Internet Assigned Numbers Authority (IANA) for standard services. But unlike port 80 (HTTP) or port 443 (HTTPS), port 737 has no official assignment. It's an empty lot in a crowded neighborhood.
The Well-Known Range
Ports 0-1023 are reserved for system services. On Unix-like systems, only processes running with root privileges can bind to these ports. This restriction exists because these ports are supposed to run trusted, standardized services—things like SSH (22), DNS (53), and SMTP (25).
When you connect to port 80, you expect HTTP. When you connect to port 22, you expect SSH. The well-known range is about predictability.
Port 737 breaks that pattern. There's nothing standardized here.
What Used to Live Here
In the early 2000s, Apple's Mac OS X used port 737 for NetInfo—a directory service for managing user accounts, network configuration, and system information across multiple Macs.1
NetInfo was inherited from NeXTSTEP (the operating system that became Mac OS X). It used RPC (Remote Procedure Call) to let Macs share directory information. Port 737 was part of that infrastructure.
But NetInfo had problems. It was proprietary, difficult to integrate with non-Apple systems, and didn't scale well. Apple replaced it with Open Directory in Mac OS X Server 10.2 (2002) and completely removed NetInfo in Mac OS X 10.5 Leopard (2007).2
When NetInfo died, port 737 became unassigned.
Why Unassigned Ports Matter
Most ports aren't assigned. Of the 65,535 possible port numbers, only a few hundred have official IANA assignments. The rest are either registered for specific applications or completely unassigned.
Unassigned ports in the well-known range are rare—and suspicious. If you see traffic on port 737 today, it's either:
- A legacy Mac running a very old version of OS X
- A custom application that chose this port arbitrarily
- Malware
Security databases flag port 737 because malware occasionally uses unassigned well-known ports. They're not commonly monitored, which makes them attractive for covert communication.3
How to Check What's Listening
If you want to see if anything is using port 737 on your system:
On Linux/macOS:
On Windows:
If something is listening and you don't recognize it, investigate. Nothing legitimate should be using this port unless you're running a very old Mac or a custom application you configured yourself.
The Gap in the Map
Port 737 is a reminder that the Internet's infrastructure isn't perfectly planned. Some ports get assigned to services that never become popular. Some services die and leave their ports behind. Some ports sit empty forever, waiting for a proposal that never comes.
The well-known range was supposed to be the definitive map—every port documented, every service standardized. But even here, there are gaps. Port 737 is one of them.
Frequently Asked Questions About Port 737
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