Port 625 has no service attached to it. No RFC defines what should run here. No protocol calls this port home. It's one of the gaps in the well-known range—a number that exists but doesn't do anything.
What "Unassigned" Means
Ports 0-1023 are called the well-known ports. They're managed by the Internet Assigned Numbers Authority (IANA), which decides what runs where. Port 22 gets SSH. Port 443 gets HTTPS. Port 80 gets HTTP.1
Port 625 was reserved but never assigned. It's been sitting empty in this range for decades.
This is different from ports in the registered range (1024-49151) or the dynamic range (49152-65535). Those ranges are meant to be flexible. The well-known range is supposed to be the stable foundation—the ports everyone knows and expects.
An empty slot here is unusual. It means IANA reserved the number but nobody ever claimed it with a protocol important enough to justify permanent assignment.
Why Security Scanners Check This Port
If you run a vulnerability scanner, it will check port 625. Not because something is supposed to be there—because nothing is.2
An unassigned port in the well-known range is suspicious when occupied. If you find a service listening on port 625, you should ask: why is it there? Why isn't it using a registered port or a dynamic port like it's supposed to?
Sometimes it's legitimate—a custom application someone configured to use this specific number. Sometimes it's a misconfiguration. Sometimes it's something worse.
The fact that security tools include port 625 in their default scan lists tells you something: empty addresses in high-value ranges attract attention. A service listening here is either lost or hiding.
How to Check What's Listening
If you want to see whether anything is actually using port 625 on your system:
Linux/macOS:
Windows:
If you see output, something is listening. You should figure out what it is and whether it belongs there.
If you see nothing, the port is doing exactly what it's supposed to do: nothing.
The Purpose of Unassigned Ports
Why does port 625 exist if nothing uses it?
Because the well-known range was designed with headroom. When the port system was created, nobody knew which services would matter. So IANA reserved the entire 0-1023 range for "important things that need stable addresses."
Some of those numbers got claimed immediately (port 25 for email, port 23 for Telnet). Others were assigned later as new protocols emerged (port 443 for HTTPS in the 1990s). And some—like port 625—never got claimed at all.
These unassigned ports serve a purpose: they're available if a genuinely important protocol emerges that needs a well-known address. They're the blank spaces in the address book, held in reserve.
But practically speaking, most new protocols don't need well-known ports anymore. Modern service discovery, DNS-based addressing, and application-layer protocols mean you don't need IANA to give you a magic number. Port 625 will likely stay empty.
What This Port Teaches
Port 625 is a reminder that the Internet was built with optimism. The architects reserved space for services that didn't exist yet—ports for protocols nobody had invented.
Some of those spaces filled. Others didn't. Port 625 sits there like an empty lot in a city that grew around it. Not useless, exactly. Just waiting for something that probably won't come.
If you find traffic on this port, investigate. The emptiness is the point. Anything occupying it should have a reason.
Frequently Asked Questions About Port 625
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