Port 588 sits in the well-known ports range (0-1023), officially assigned by the Internet Assigned Numbers Authority (IANA) to a service called "CAL." Despite this official status, it's one of the Internet's quieter addresses—reserved but rarely used.
What CAL Is (Probably)
The IANA registry lists port 588 as assigned to "CAL" for both TCP and UDP, but the protocol's purpose isn't widely documented. Some sources mention associations with IBM LM MT Agent and IBM Application software, suggesting it may have been used for proprietary remote procedure calls or calendar access functions.12
The name "CAL" could refer to Calendar Access Protocol or a related service, but without accessible RFC documentation or active implementations, the protocol's original purpose remains unclear.
The Well-Known Range
Ports 0-1023 are the well-known ports—addresses reserved by IANA for standardized services. Getting a port number in this range requires formal assignment, typically backed by an RFC or substantial technical documentation.
Port 588 has this official assignment. Someone, at some point, requested it. Someone approved it. But unlike its neighbors—ports that carry DNS, HTTP, or email—port 588 never became part of the Internet's everyday traffic.
What This Means in Practice
If you see port 588 in use on your network, it's worth investigating. It could be:
- Legacy IBM software from the 1990s or early 2000s
- A proprietary application using the officially assigned port
- An unofficial use by software that chose an obscure assigned port
- Potentially suspicious activity masquerading as legitimate traffic
The official assignment doesn't mean the port is commonly used—just that it's reserved.
Checking What's Listening
On Linux or macOS:
On Windows:
If something is listening on port 588, the process ID will tell you what software is using it.
Why Unassigned and Unused Ports Matter
The port number system has 65,535 addresses per protocol. Not all of them are famous. Not all of them see daily use. But each assigned port represents someone's intention—a protocol designed, a service planned, a need identified.
Port 588 is a reminder that the Internet's infrastructure includes not just the ports we use every day, but also the ones we reserved just in case. Some became critical. Some became forgotten. Both are part of the story.
Related Ports
- Port 587 — SMTP message submission, one number lower and vastly more famous
- Port 500 — IKE (Internet Key Exchange) for VPN connections
- Port 443 — HTTPS, the encrypted web
Frequently Asked Questions
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