Port 624 sits in an unusual position: officially assigned, rarely used. IANA registered it for "cryptoadmin"—cryptographic administration services—but if you scan networks today, you'll almost never find anything listening here.1
What Lives Here (Officially)
Port 624 is assigned to a service called cryptoadmin (Crypto Admin) for both TCP and UDP.2 The service was meant for cryptographic administration tasks—managing encryption keys, certificates, or cryptographic operations across a network.
The problem: the service never took hold. No widely-deployed tool claimed this port. No major vendor built cryptoadmin into their products. The registration exists, but the ecosystem moved on.
The Well-Known Range
Port 624 falls in the well-known port range (0-1023), which IANA reserves for system services and standardized protocols. These ports require root or administrator privileges to bind on Unix-like systems—a security measure to prevent unauthorized services from masquerading as trusted ones.3
Being in this range means port 624 was considered important enough to warrant official assignment. Someone believed cryptographic administration deserved a permanent home. But importance in theory doesn't guarantee adoption in practice.
What Actually Uses It
In the wild, port 624 occasionally appears in three contexts:
Apple legacy services — Older documentation mentions port 624 for Apple Network Server (ANS) time protocol or Macintosh Manager.4 These were 1990s-era services that have since been deprecated or replaced.
Security scans — Some security databases flag port 624 as historically associated with trojan activity.5 This doesn't mean the port itself is dangerous—just that malware authors sometimes repurpose quiet, unused ports for command-and-control traffic.
Nothing — Most commonly, port 624 is closed. Firewalls block it by default. No service binds to it. It exists in the registry and nowhere else.
Why Empty Ports Matter
The Internet's port system has 65,535 possible addresses per protocol (TCP and UDP). Not all of them get used. Some, like port 624, were claimed early with good intentions but never found their purpose.
These dormant ports serve as a reminder: the protocol doesn't guarantee adoption. Standards bodies can assign numbers, but they can't force the industry to build services around them. Port 624 has a name, an official assignment, and a place in the registry. What it doesn't have is traffic.
How to Check What's Listening
If you want to see whether anything is actually using port 624 on your system:
On Linux or macOS:
On Windows:
Most of the time, you'll get nothing back. That's normal. Port 624 is assigned, but empty.
Security Considerations
Because port 624 is rarely used for legitimate services, any traffic here warrants investigation. If you see something listening on port 624 that you didn't intentionally configure:
- Check what process is bound to the port
- Verify it's not malware repurposing the unused port
- Consider blocking port 624 at the firewall if you're not using it
Unused ports are attractive to attackers precisely because they're unused—less likely to be monitored, less likely to trigger alerts.
The Lesson
Port 624 is a registered ghost. It has official status but no meaningful presence. The cryptoadmin service that was supposed to live here never materialized. What remains is an empty room in the Internet's address space—reserved, documented, and silent.
That's not a failure. It's just the reality of protocol evolution. Not every good idea gets implemented. Not every assignment gets used. Port 624 exists so that if cryptoadmin ever does arrive, it has somewhere to go.
So far, it hasn't.
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