What Port 89 Does
Port 89 is assigned to the SU/MIT Telnet Gateway (service name su-mit-tg), a protocol that allowed users to establish Telnet sessions between networks running different protocols. "SU" is Stanford University. "MIT" is the Massachusetts Institute of Technology. The gateway sat between them, translating, so that a researcher at one institution could reach a machine at the other.
The protocol is registered for both TCP and UDP. It has been obsolete for decades. No known implementations are running today.
But the registration remains in the IANA port registry, exactly where it was placed in the 1980s.1
How It Worked
In the late 1970s and early 1980s, different institutions ran different networking stacks. Stanford's AI Lab ran WAITS on PDP-10 machines. MIT had its own systems. The ARPANET connected them, but protocol differences meant that a simple Telnet connection from one campus to the other wasn't always straightforward.
The SU/MIT Telnet Gateway solved this by acting as a translator. A user would connect to port 89 on the gateway machine, and the gateway would handle the protocol negotiation needed to bridge the connection to the other side. It was infrastructure for a problem that no longer exists: the Internet hadn't yet converged on TCP/IP as the universal language.
The Person Behind the Port
Port 89 was registered by Mark Crispin (1956-2012), listed as [MRC] in the IANA registry.2
Crispin was a systems programmer at Stanford University from 1977 to 1988. He wrote or rewrote most of the ARPANET protocol suite for Stanford's WAITS operating system. He developed the NCP (Network Control Program) implementation for the PDP-10. He built the TOPS-20 mail system.
And then, between 1985 and 1988, he invented IMAP, the Internet Message Access Protocol, which runs on port 143.3 IMAP changed email forever. Before IMAP, you downloaded your email and it lived on your machine. After IMAP, your email lived on a server and you could access it from anywhere. Every modern email client, every webmail interface, every phone checking your inbox: that paradigm traces back to Mark Crispin at Stanford.
He also designed the SUPDUP protocol on port 95, a display-oriented terminal protocol described in RFC 734 that offered richer terminal capabilities than Telnet.4
Three ports in the well-known range (0-1023), all registered by the same person. Port 89, port 95, port 143. Three fingerprints on the nervous system of the Internet.
Crispin moved to the University of Washington in 1988, where he spent twenty years developing UW IMAP and co-creating the Pine email client. He was reportedly still running TOPS-20 systems at his home in 2009. He died on December 28, 2012, in Poulsbo, Washington. In 2013, his alma mater, Stevens Institute of Technology, awarded him a posthumous Distinguished Alumni Award.5
Port 89 Today
The SU/MIT Telnet Gateway protocol is completely defunct. No implementations exist. The problem it solved, bridging incompatible network protocols between institutions, was solved permanently when TCP/IP became the universal standard.
Port 89 occasionally appears in firewall logs, which can cause confusion. Any traffic on port 89 today is not the SU/MIT Telnet Gateway. It could be a misconfigured application, a port scan, or something else entirely.
Checking What's Listening on Port 89
Linux:
macOS:
Windows:
If something is listening on port 89, investigate it. The legitimate protocol for this port has been dead for decades.
Security
Port 89's original protocol, like all Telnet-based services, transmitted everything in plaintext. No encryption, no integrity checking, no protection against eavesdropping. This was normal in the late 1970s. It would be unacceptable today.
Because port 89 is in the well-known range (0-1023), binding to it on Unix systems requires root privileges. Any unexpected traffic on this port should be treated with suspicion. Port scanners routinely probe well-known ports regardless of whether they have active services, so seeing connection attempts to port 89 in your firewall logs is not unusual, but it is worth noting.
The Quiet Ports
Port 89 belongs to a category of ports that carried something vital for a brief window of Internet history, then fell silent. The protocol it served solved a real problem for real people: researchers who needed to reach machines across institutional boundaries when the network was still figuring out how to be one network instead of many.
The fact that this problem was solved so thoroughly that the gateway became unnecessary is itself a kind of triumph. Port 89 is a monument to a bridge that worked so well it made itself obsolete.
Frequently Asked Questions
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