Port 193 sits in the well-known ports range with an official assignment, but it carries something rare in the Internet's architecture: a protocol with no history.
What Port 193 Is Assigned To
Port 193 (both TCP and UDP) is officially registered with IANA to SRMP — the Spider Remote Monitoring Protocol.12 It's classified as a System Port, meaning it's in the privileged range (0-1023) reserved for standardized services.
The assignment appears in RFC 1700, the last comprehensive list of assigned port numbers published in 1994.3 When RFC 1700 was obsoleted by RFC 3232 in 2002, IANA moved to maintaining living documents online, and port 193's assignment to SRMP remained.
The Problem: Nobody Knows What SRMP Actually Was
Here's what's strange: despite being officially assigned a well-known port number, there is virtually no documentation about what the Spider Remote Monitoring Protocol actually did.45
No RFC defines it. No technical specifications survive. No implementation details exist in any accessible archive. The protocol has a name, a port number, and a contact person (Ted J. Socolofsky), but no visible history of actual use.
It's a ghost in the registry—formally present but functionally absent.
What Well-Known Ports Mean
Port 193 sits in the range 0-1023, the System Ports or Well-Known Ports.6 This range is managed by IANA and reserved for standardized protocols. You need root or administrator privileges to bind a service to these ports.
Well-known ports were meant to create a stable addressing system for fundamental Internet services. Port 22 is always SSH. Port 80 is always HTTP. Port 443 is always HTTPS.
Port 193 was meant to always be SRMP—except SRMP itself seems to have barely existed.
Why This Port Matters
The existence of port 193 reveals something important about how the Internet's numbering system works: once assigned, ports are rarely revoked.
IANA doesn't actively reclaim port numbers from defunct or never-implemented protocols. The registry grows but rarely shrinks. This means the well-known ports range contains numerous assignments like port 193—formally reserved, functionally unused, holding space for services that faded or never materialized.
This isn't a bug. It's a feature of stable addressing. The cost of reclaiming port 193 and potentially breaking something, somewhere, that might depend on it is higher than the cost of just leaving it assigned.
Checking What's Actually on Port 193
If you want to see if anything is listening on port 193 on your system:
Linux/Mac:
Windows:
Network scanning with nmap:
In most cases, you'll find nothing. Port 193 is almost certainly closed on your machine and on most machines you'll ever encounter.
Security Considerations
Because port 193 is largely unused, it's sometimes co-opted by malware or unauthorized services precisely because it's unexpected.7 If you do find something listening on port 193, it warrants investigation—it's probably not SRMP.
Attackers occasionally use obscure assigned ports because security tools focus on common ports (80, 443, 22, etc.). An open port 193 won't necessarily trigger alerts, making it useful for covert communication.
The Internet's Fossil Record
Port 193 is part of the Internet's fossil record—layers of history preserved in registries, most of it still functional, some of it vestigial.
The port sits there, officially assigned, waiting for traffic that will probably never come. It's a reminder that the Internet's addressing system isn't just a technical specification—it's an archive of intentions, some realized, some abandoned, some never quite begun.
Spider Remote Monitoring Protocol got a port number. That's more permanence than most software ever receives.
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