Port 923: Unassigned — A door in the well-known range with no service behind it

Port 923 has no official assignment. It sits in the well-known port range—the first 1,024 ports (0-1023) that IANA reserves for standardized Internet services—but nobody ever claimed it.

What Does "Unassigned" Mean?

The well-known port range exists to provide stable, universal addresses for critical Internet protocols. When you connect to port 80, you expect HTTP. Port 443 means HTTPS. Port 25 is SMTP for email. These assignments are registered with IANA and documented in RFCs so that every device on the Internet knows what to expect.

Port 923 has none of this. It's unassigned—available for use, but not reserved for any particular service.¹

The Well-Known Port Range (0-1023)

Ports 0 through 1,023 are called "well-known ports" or "system ports." On Unix-like operating systems, only processes running with superuser privileges can bind to these ports. This restriction exists because well-known ports are supposed to run trusted services that everyone depends on.

Port 923 falls into this range, which means:

• On Linux, macOS, and other Unix systems, you need root privileges to run a service on port 923
• The port was reserved in case a future Internet standard needed it
• No RFC or IANA assignment ever came

The Internet Assigned Numbers Authority maintains the official registry of port assignments.² Port 923 appears in that registry only as part of an unassigned block.

Does Anything Actually Use Port 923?

Officially? No.

Unofficially? Probably not much. Some older documentation mentions that Mac OS X used port 923 for RPC-based services like NetInfo, which was a directory service on legacy macOS systems.³ NetInfo is long deprecated—Apple replaced it with Open Directory in Mac OS X 10.5 (2007) and hasn't used it since.

You might find custom applications or internal services using port 923, but there's no standard protocol you can expect to find there.

How to Check What's Listening on Port 923

If you want to see if anything is actually using port 923 on your system:

On Linux or macOS:

sudo lsof -i :923

On Windows:

netstat -ano | findstr :923

If nothing returns, the port is closed. If something is listening, you'll see the process name and ID. At that point, you can decide if it's something you recognize or something suspicious.

Why Unassigned Ports Matter

The existence of unassigned ports isn't a problem—it's actually useful. Not every future Internet protocol was anticipated in the 1980s when IANA started assigning ports. Leaving some ports unassigned means there's room for new standards.

But port 923 has been unassigned for decades. At this point, it's unlikely to ever receive an official assignment. The Internet has moved on. Modern protocols increasingly use dynamically allocated ports or multiplex multiple services over a single port (like HTTP/2 and HTTP/3 do over port 443).

The well-known port range is mostly full. The unassigned ports that remain—like 923—are the quiet gaps in a neighborhood that was planned but never completely filled.

What Happens If You Use Port 923?

Nothing stops you from running a service on port 923. It's unassigned, not forbidden. If you're building a custom application and need a port number, you could choose 923.

But you probably shouldn't.

Better practice: use the registered port range (1024-49151) or the dynamic/ephemeral range (49152-65535). Those ranges exist for custom and temporary services. The well-known port range—even the unassigned parts of it—carries an expectation of standardization. Using it for a custom service creates confusion.

If you need a port, pick one that's meant for what you're doing.

The Story of the Gaps

Port 923 is one of many unassigned ports scattered across the well-known range. These gaps exist for a few reasons:

• Foresight: IANA left space for future protocols that might need a permanent home
• Abandonment: Some ports were reserved but never used, and the proposals died
• Politics: Occasionally, competing standards fought over a port number and nobody won

Port 923 doesn't have a dramatic story. It's just an address nobody needed badly enough to claim.

And that's fine. Not every door needs to be in use. The Internet works because most ports do their jobs. The unassigned ones wait quietly, just in case.