Protocol: adapt-sna (Network Software Associates)
Port Type: Registered (1024-49151)
Transport: TCP and UDP
What This Port Does
Port 1365 is officially registered to adapt-sna, a protocol developed by Network Software Associates for connecting MS-DOS and OS/2 personal computers to IBM mainframe systems over asynchronous (phone) lines.12
This wasn't about accessing websites or sending email. This was about letting a PC at someone's desk function as an IBM 3270 terminal—the green-screen terminals that dominated corporate computing in the 1980s and 1990s—while still being a fully functional personal computer.
The Problem It Solved
In the early 1990s, corporate computing lived in two worlds. The important data—payroll, inventory, customer records—lived on IBM mainframes running SNA (Systems Network Architecture).3 But employees were starting to get personal computers on their desks.
The problem: how do you let that PC access the mainframe data without installing expensive dedicated terminal connections?
Network Software Associates created AdaptAsync, a hardware-software solution that let PCs connect to IBM hosts over regular phone lines (asynchronous connections) while appearing to the mainframe as proper SNA devices.4 The adapt-sna protocol on port 1365 handled this communication.
What SNA Was
Systems Network Architecture was IBM's proprietary networking architecture, introduced in 1974.3 Before TCP/IP and the Internet won, SNA was how large organizations networked their computers. It was hierarchical—mainframes at the center, terminals at the edges—and it required everything to speak IBM's language.
SNA wasn't designed for the peer-to-peer, interconnected world we have now. It was designed for a world where "the computer" meant "the mainframe in the data center" and everything else was just a way to talk to it.
Why You Probably Won't See This Port Today
Port 1365 is a fossil. IBM mainframes still exist, and some organizations still run SNA networks, but the adapt-sna protocol solved a problem that doesn't really exist anymore. Modern systems either:
- Access mainframe data through web interfaces
- Use terminal emulation software that doesn't need special protocols
- Have migrated away from mainframe-centric architectures entirely
The port remains in the IANA registry because it was officially registered decades ago, but active traffic on port 1365 is rare.
How to Check What's Using This Port
On Linux or macOS:
On Windows:
If you see something listening on port 1365, you've either found a very old system still running legacy mainframe connectivity software, or something else has repurposed this registered port for its own use.
The Registered Port Range
Port 1365 falls in the registered ports range (1024-49151). These ports are assigned by IANA to specific services, but the assignments aren't enforced—any application can technically use any registered port. The registry exists to prevent conflicts, not to grant exclusive rights.
The difference between registered ports and well-known ports (0-1023) is mainly historical. Well-known ports required root privileges to bind on Unix systems, which provided some security. Registered ports don't have that protection, but they serve as a coordination mechanism: if you're building a network protocol, you can request a port number so other developers know not to use it.
Why This Port Matters
Port 1365 matters because it represents a moment in computing history—the awkward transition when personal computers were everywhere but the valuable data was still locked in mainframe systems designed for a different era.
Someone at Network Software Associates saw that gap and built a bridge. They registered port 1365 with IANA. Companies bought the software. Employees used it to access mainframe data from their desktop PCs without knowing or caring about the protocol making it work.
Then the Internet happened. Client-server architecture replaced hierarchical mainframe models. Web browsers replaced terminal emulators. And port 1365 became a footnote—a number in a registry, a reminder that before IP won, there were dozens of other protocols solving dozens of other problems.
Most of them are gone now. But their port numbers remain.
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