Port 204 (both TCP and UDP) is assigned to AppleTalk Echo, a diagnostic protocol from a networking suite that most of the Internet has forgotten. It was part of AppleTalk, Apple's proprietary networking system released in 1985 for the original Macintosh.1
AppleTalk is dead. Apple dropped support in 2009 when Mac OS X v10.6 shipped.2 But port 204 remains officially assigned to it in IANA's registry—a permanent grave marker for a protocol that once connected millions of Macs.
What AppleTalk Echo Did
The AppleTalk Echo Protocol (AEP) was simple: it let one Mac ask another Mac "are you there?" across a network.3
You'd send an AEP packet to another node. If that node existed and could respond, it would flip the packet into an AEP Reply and send it back. You'd know the remote system was alive and could open a session.
It was a reachability test. A heartbeat check. The digital equivalent of knocking on a door and waiting to hear footsteps.
This was 1985. Most people didn't have networks. The ones who did were wrestling with manual IP configuration and routing tables. AppleTalk was revolutionary because it was plug-and-play—you connected a Mac to the network and it just worked. No configuration. No central server. No manual addressing.4
The Story of AppleTalk
In the early 1980s, Apple engineers preparing to launch the Macintosh knew networks would matter. They wanted a network that felt like the Mac itself: simple, intuitive, seamless.5
They built AppleTalk. Released in 1985. By 1988, AppleTalk had three times more installations than any other networking vendor.6 It wasn't just for Macs—Apple released a PC card in 1987 so DOS machines could join AppleTalk networks and print to LaserWriter printers.7
AppleTalk Phase 1 supported one network segment and 127 devices. Phase 2, released in 1988, expanded this dramatically. AppleTalk became the standard for connecting Macs, printers, and file servers throughout the 1980s and 1990s.
But TCP/IP won. The Internet won. By the time Steve Jobs returned to Apple, AppleTalk was legacy infrastructure. Mac OS X made TCP/IP the default. By 2009, AppleTalk was gone entirely.
Why Port 204 Matters
Port 204 is a fossil. It's assigned to a protocol that almost nobody uses anymore, running on a network stack that was discontinued 15 years ago.
But it remains in the registry because the Internet has a long memory. Port assignments aren't reused lightly. Systems that still run old software, emulators that recreate vintage networks, historians who study how networking evolved—they all depend on these assignments staying stable.
Port 204 is history written in numbers. It's proof that Apple once built not just computers but the entire network to connect them. It's a reminder that before TCP/IP became universal, there were other visions of how computers should talk to each other.
Some of those visions were beautiful. Some were better in certain ways. But only one became the Internet.
How to Check What's Listening on Port 204
On Linux or macOS:
On Windows:
You probably won't find anything. Port 204 is a ghost port now—assigned but mostly silent.
Related Ports
AppleTalk used several ports in the well-known range:
- Port 201: AppleTalk Routing Maintenance
- Port 202: AppleTalk Name Binding
- Port 203: AppleTalk Unused
- Port 204: AppleTalk Echo
- Port 206: AppleTalk Zone Information
All of them are memorials now. All of them remember a time when Apple thought different about networking too.
Frequently Asked Questions
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