Port 596 sits in an unusual category: it's an officially assigned well-known port, but the protocol it was meant for has essentially disappeared from the Internet.
What Runs on Port 596
Port 596 was registered with IANA for SMSD, a network management protocol that operated on both TCP and UDP. The registration lists Wayne Barlow from DEC/Unix as the contact.12
That's about all we know.
The Ghost Protocol
SMSD appears to have been a Digital Equipment Corporation (DEC) protocol from the Unix era—likely from the 1980s or early 1990s. But unlike protocols that became Internet standards with published RFCs and detailed documentation, SMSD left almost no trace.
No RFC defines it. No technical specification is publicly available. The protocol itself has been forgotten, even though its port number remains officially reserved in IANA's registry.
This happens more often than you'd think. Port 596 is one of hundreds of officially assigned ports whose protocols never became widely adopted, or were used internally by companies that no longer exist, or simply faded away as technology evolved.
What This Port Means Today
In practice, port 596 is essentially unassigned. The SMSD protocol isn't running on modern networks. The port number sits reserved but unused—a historical artifact in the port registry.
If you find something listening on port 596 on your network, it's almost certainly not SMSD. It's either:
- A modern application using an "available" port it found
- Malware trying to hide in an obscure port number
- A custom internal service that chose 596 arbitrarily
Why Reserved Ports Matter
Port 596 represents an interesting quirk of Internet governance. Once IANA assigns a well-known port number (0-1023), it stays assigned even if the protocol dies. The registry is permanent.
This is actually by design. Port numbers are a finite resource (only 1,024 well-known ports exist), and IANA wants to avoid reusing numbers that might cause conflicts with legacy systems still running old software.
So port 596 sits there—officially assigned to a protocol that no one uses, unable to be reassigned, a small monument to a piece of networking history that vanished before the Internet became what it is today.
Checking What's Actually Running
If you want to see if anything is actually listening on port 596 on your system:
Linux/Mac:
Windows:
If something appears, it's almost certainly not SMSD.
The Well-Known Ports Category
Port 596 belongs to the well-known ports range (0-1023), which IANA reserves for standardized system services. These ports require root/administrator privileges to bind to on most systems—a security measure to prevent unprivileged programs from impersonating critical services.
But as port 596 demonstrates, "well-known" doesn't mean "well-used." The registry contains many ghosts.
Related Ports
Other obscure well-known ports from the early Internet:
- Port 95: SUPDUP (a remote login protocol that predated SSH)
- Port 158: PCMAIL-SRV (an early email system)
- Port 315: DPSI (a protocol whose purpose is lost to time)
Frequently Asked Questions
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