# WHATPORT(4)

## NAME

port 4 — Unassigned — Unassigned at IANA — no registered service, nothing standard listens here.

Protocol: TCP  
IANA registry

## DESCRIPTION

IANA lists port 4 as Unassigned for both TCP and UDP, with no assignee and no history of a well-known service. No mainstream daemon binds it by default. Anything listening on 4 is custom software or a deliberately unusual configuration, and the only way to know what it is, is to fingerprint the listener and check which process owns the socket locally.

## EXPOSURE

**Expose with care.** An unregistered low port that is open is an unidentified service; treat it as unknown until you have named the process.

## CHECK

```sh
sudo lsof -nP -iTCP:4 -sTCP:LISTEN
```

## SEE ALSO

- iana — [IANA Service Name and Transport Protocol Port Number Registry](https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml) (iana.org)

---

HTML: <https://whatport.net/4/> · JSON: <https://whatport.net/4.json> · full dataset: <https://whatport.net/ports.json>