Port 3556: sky-transport — A Name Without a Story

What the Registry Says

IANA lists port 3556 as sky-transport — Sky Transport Protocol — registered for both TCP and UDP in July 2002.¹ That's the complete official record. No RFC. No public specification. No known vendor or project that claimed authorship.

It sits in the registered port range (1024–49151), which means someone formally requested the assignment from IANA at some point. That's more than most obscure ports get. But the registration alone doesn't tell you what the protocol actually does, and in this case, nothing else does either.

The Registered Port Range

Ports 1024–49151 were designed for services that need a stable, reserved number — things like databases, game servers, and enterprise software that can't just pick an arbitrary port and hope for the best. To claim one, you historically submitted a request to IANA with a service name and contact.

The problem: registration required very little. A name. An email. An intention. The Internet was moving fast in 2002, and many entries in this range were filed by products that never shipped, companies that folded, or protocols that were superseded before they were ever deployed. Port 3556 appears to be one of those.²

What's Actually on Port 3556 Today

In practice, port 3556 is unoccupied on most systems. If you see traffic on it, the most likely explanation is:

• Custom application: Internal software at a company that picked this port for its own purposes
• Port scanner noise: Background Internet scanning that probes all ports looking for open services
• Misconfiguration: Something meant to run on a different port

To check what's actually listening on port 3556 on your system:

macOS / Linux:

lsof -i :3556

Windows:

netstat -ano | findstr :3556

Linux (alternative):

ss -tlnp sport = :3556

If nothing is listening, the port is closed and unreachable from outside your machine — which is the normal and expected state.

Why This Matters

The registered port range has over 48,000 possible slots. Thousands of them look like port 3556: names that appear in the IANA registry, point to no specification, and show up in no known software. They're archaeological artifacts of a period when claiming a port number felt like staking a claim on something valuable.

For network administrators, the practical lesson is simple: an IANA registration tells you a port has a name, not that anything is actually using it. When you see unexpected traffic on port 3556 — or any obscure registered port — the registry is rarely the right place to look. The right place is the process listening on the other end.