What Runs on Port 626
Port 626 (UDP) was officially assigned by IANA to serialnumberd, an Apple Mac OS X Server service that ran from approximately 2004 to 2009 (Mac OS X Server versions 10.3 through 10.6).12
The service had one job: prevent the same server serial number from being used on multiple machines on the same local network.
How It Worked
Every five minutes, serialnumberd sent a UDP packet on port 626 to every active network interface. The packet announced: "I am using serial number X."3
If another Mac OS X Server on the same LAN responded with the same serial number, the system knew someone was violating the license agreement. Server-specific features would refuse to run.
This wasn't DRM in the modern sense—no phone-home to Apple, no Internet connection required. Just local network broadcasts checking if you were running the same serial number twice in the same building.
Why This Port Exists
In the mid-2000s, Apple sold Xserve rack-mounted servers and Mac OS X Server software. Organizations might run multiple servers on the same network. Serial number checking prevented someone from buying one license and installing it on ten machines.
The check was simple: broadcast your serial number every five minutes. Listen for anyone else broadcasting the same number. If detected, refuse to work.
It was license enforcement designed for a time when "the server" meant physical hardware in your own data center, not a virtual machine in someone else's cloud.
Current Status
Serialnumberd is gone. Apple discontinued Xserve in 2011. Mac OS X Server became macOS Server, then eventually just a simple app with minimal features. The five-minute broadcasts stopped.
Port 626 remains assigned to serialnumberd in the IANA registry, but nothing listens there anymore—unless you're running a very old Mac OS X Server installation.
The Range This Port Belongs To
Port 626 sits in the well-known ports range (0-1023), which requires IANA assignment and typically needs root/administrator privileges to bind to. This is unusual for a vendor-specific service like serialnumberd, which would normally be assigned a registered port (1024-49151).
The assignment suggests Apple requested it early, when well-known port assignments were easier to obtain.
Checking What's Listening
On macOS or Linux:
On Windows:
You'll almost certainly find nothing. This port is a museum piece.
Why Unassigned Ports Matter
Wait—port 626 is assigned. But most ports in the well-known range are not. The unassigned ones matter because they represent possibility: space for future protocols, room for experimentation, gaps where new standards can emerge.
Port 626 shows what happens when a protocol dies but its port number lives on. The registry is a graveyard as much as a directory. Some ports carry traffic for decades. Others, like this one, had a brief moment of purpose and then went silent.
The serial number police stopped patrolling. The port remains reserved.
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