Port 373 tells a strange story about the ephemeral nature of network protocols. This well-known port was officially assigned to a company that no longer exists, for a service lost to time. And in the same breath, network documentation lists it for an entirely different protocol that Apple killed over a decade ago.
Both assignments are dead. The port remains.
What Port 373 Was Officially For
According to IANA's official registry, port 373 is assigned to legent-1, a service from Legent Corporation.1
Legent Corporation was formed in 1989 by merging two mainframe systems management software companies: Duquesne Systems and Morino Associates.2 They built software that helped enterprises manage their mainframe computers, workstations, and early client/server systems. Products for network and distributed management, file transfers across different operating systems, the kind of infrastructure software that kept 1990s data centers running.
In 1995, Computer Associates acquired Legent.3 The port assignment survived in IANA's registry. The service it was meant for vanished into the absorption of one software company by another.
What exactly ran on port 373 for Legent? The documentation doesn't say. The people who knew are scattered. The code is probably gone. What remains is a line in a registry: port 373, legent-1, assigned.
What Port 373 Was Unofficially For
Multiple network port databases also list port 373 as used by AppleTalk Session Protocol (ASP).4
AppleTalk was Apple's proprietary networking suite, designed in the 1980s to let Macintosh computers talk to each other and share files. ASP sat in the session layer, managing connections between clients and servers. When a Mac wanted to access an AppleShare file server, ASP on port 373 established and maintained that session.5
ASP provided the foundation for AFP (AppleTalk Filing Protocol), which handled all the actual file sharing. The protocol was asymmetrical - all communication initiated by the workstation, responded to by the server. It extended AppleTalk Transaction Protocol (ATP) into the session layer, providing reliable ordered delivery.
AppleTalk had no encryption, no authentication mechanisms to speak of. It assumed every device on the local network was trusted.6 That assumption held in the small offices and schools where Macs lived in the 1980s and 1990s. It didn't hold when the Internet became ubiquitous.
When It Died
Apple deprecated AppleTalk with the release of Mac OS X 10.6 Snow Leopard in 2009.7 TCP/IP had won. Everything moved to TCP/IP. The autoconfiguration features that made AppleTalk elegant were reimplemented in Bonjour. File sharing moved to SMB. By the 2020s, AppleTalk exists only in the memory of sysadmins who managed Mac networks in the 1990s.
Legent died earlier, absorbed into Computer Associates in 1995. The software products were folded into CA's portfolio, rebranded, eventually discontinued or replaced. The port assignment remained in IANA's registry because nobody bothered to remove it.
The Conflict
So which was it? Legent or AppleTalk?
The official answer: Legent. IANA assigned port 373 to legent-1. That's the authoritative record.
The practical answer: Maybe both. Port assignments weren't enforced like laws. Companies and protocol designers sometimes picked ports without checking, or used unassigned ports, or ignored assignments entirely. It's possible Legent's software and AppleTalk both used port 373, on different networks, never knowing the other existed.
Or maybe the AppleTalk documentation is wrong. Maybe someone writing a port database confused 373 with another port. Maybe ASP used a different port and the error propagated through decades of copy-pasted port lists.
The truth is lost. Both uses are dead. The conflict doesn't matter anymore.
What Range This Port Belongs To
Port 373 is in the well-known ports range (0-1023). These are the System Ports, assigned by IANA through formal procedures requiring IETF Review or IESG Approval.8 This range was meant for standardized services that everyone needed to interoperate - SSH on 22, HTTP on 80, HTTPS on 443.
Port 373 represents the messier reality: assignments to corporate products that disappeared, conflicts between official assignments and documented usage, ghost services that nobody remembers.
Should You Care About Port 373?
No. Unless you're running a museum-piece Mac network from 1995 or you've discovered a cache of Legent software in a data center basement, port 373 carries nothing.
If something is listening on port 373 on your network, it's either:
- Legacy software nobody's updated in decades
- A service that picked the port arbitrarily
- Malware (some Trojan activity has been documented on this port)9
Check what's listening:
If you find something, figure out what it is. The legitimate uses for port 373 are extinct.
Why This Port Matters
Port 373 is a reminder that the Internet's port system is not a clean, logical registry maintained with perfect foresight. It's an archaeological record of networking history - failed protocols, acquired companies, good ideas replaced by better ones, conflicts nobody resolved because everyone moved on.
The well-known ports range is full of ghosts like this. Assigned to services that no longer exist, for companies that were absorbed, for protocols that lost to TCP/IP. Each one represents someone solving a problem in the 1980s or 1990s, getting an official assignment, building something that worked for a while.
And then the world moved on. The port numbers remain, carved into IANA's registry like headstones.
Port 373 carried sessions between Macs sharing files in small offices. Port 373 carried something for Legent's mainframe management software in enterprise data centers. Both networks are gone. The port is empty now. The number persists.
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