What Lives Here
Port 203 (TCP/UDP) is officially assigned to "at-3" in the IANA registry, designated as "AppleTalk Unused."1 That's not a typo. The official name for this port is literally "AppleTalk Unused."
It was reserved as part of Apple's AppleTalk networking protocol suite but never actually assigned to a specific service. It's a ghost — officially claimed territory that was never inhabited.
The AppleTalk Context
To understand port 203, you need to understand what AppleTalk was.
In 1985, Apple introduced AppleTalk as a proprietary networking protocol suite designed for simplicity.2 The vision was ambitious: networking that required no complex setup, no centralized routers, no dedicated servers. Users could share files and printers seamlessly across small workgroups without needing to understand what was happening underneath.
AppleTalk claimed a series of ports in the well-known range:
- Port 201: at-rtmp (AppleTalk Routing Maintenance)
- Port 202: at-nbp (AppleTalk Name Binding)
- Port 203: at-3 (AppleTalk Unused)
- Port 204: at-echo (AppleTalk Echo Protocol)
Port 203 was reserved alongside its siblings, presumably for future expansion or a planned service that never materialized. While the other ports carried real protocols — routing tables, name lookups, echo responses — port 203 remained silent.
What AppleTalk Was Trying to Do
AppleTalk wasn't just a networking protocol. It was a philosophy about how computers should communicate.
The Zone Information Protocol (ZIP) let administrators organize networks into human-readable zones like "Accounting Department" or "Design Studio."3 The Name Binding Protocol (NBP) meant you could connect to "Kirk's LaserWriter" instead of "192.168.1.47." The Routing Maintenance Protocol kept networks connected without requiring specialized knowledge.
These weren't just technical features. They were attempts to make networking humane.
And port 203? It was held in reserve, waiting for the next piece of that vision.
What Happened
Apple released a major redesign in 1989 called AppleTalk Phase II, making the protocol more generic and scalable.2 But the computing world was already moving toward TCP/IP. The Internet was growing. Open standards were winning.
By 2009, Apple dropped AppleTalk support entirely in Mac OS X v10.6.2 The protocol that had powered Macintosh networking for over two decades was gone.
Port 203 never got its moment. The service it was reserved for never arrived. And now the entire protocol suite it belonged to is obsolete.
The Well-Known Range
Port 203 sits in the well-known ports range (0-1023), which are system ports typically assigned by IANA through formal procedures.4 Getting a port in this range means going through "IETF Review" or "IESG Approval" — official processes that reserve numbers for protocols the Internet community has vetted.
This makes port 203 unusual. It's not unassigned. It's not available. It's officially claimed by a protocol that no longer exists, for a service that never existed, and it will probably stay that way forever.
IANA doesn't casually reclaim port assignments. Even when protocols die, their port numbers often remain reserved as historical markers.
Security Notes
Some security databases flag port 203 as potentially associated with malware.5 This doesn't mean the port itself is dangerous — it means that at some point, malicious software used this obscure, unused port for communication, probably because no legitimate service was listening there.
This is common with abandoned ports. When a port number has no official service running, it becomes attractive to malware authors looking for covert communication channels.
If you see unexpected traffic on port 203, investigate it. Nothing legitimate should be using this port anymore.
How to Check What's Listening
On Linux or macOS:
On Windows:
You shouldn't find anything. If you do, figure out what it is.
Why Unassigned (or Unused) Ports Matter
Port 203 is a reminder that the port number system isn't just a technical registry — it's a historical record.
Every number between 0 and 1023 was assigned with intention. Some became critical infrastructure (port 80, port 443). Some became obsolete (port 70 for Gopher). And some, like port 203, never became anything at all.
But the number remains. Officially claimed. Marked "Unused." A placeholder for a vision of networking that never fully materialized.
The Internet is built on layers of abandoned plans, protocols that died, and ports that never found their purpose. Port 203 is one of them.
Related Ports
- Port 201: AppleTalk Routing Maintenance (at-rtmp)
- Port 202: AppleTalk Name Binding (at-nbp)
- Port 204: AppleTalk Echo Protocol (at-echo)
- Port 206: AppleTalk Zone Information (at-zis)
All obsolete now. All part of the same story.
Frequently Asked Questions
Was this page helpful?