# WHATPORT(9900)

## NAME

port 9900 — IUA — ISDN Q.921 User Adaptation — IUA, the SIGTRAN protocol that backhauls ISDN Q.921 signalling over IP — normally SCTP, with TCP and UDP registered too.

Protocol: TCP/UDP  
IANA registry

## DESCRIPTION

RFC 4233 defines IUA, which carries ISDN Q.921-user messages such as Q.931 call signalling between a Signaling Gateway and a Media Gateway Controller over IP. The RFC states the registered port for IUA is 9900, and IANA lists it on SCTP, TCP, and UDP; SCTP is the intended transport, so a TCP listener on 9900 is often something else entirely. You meet IUA inside telecom cores and in softswitch stacks bridging legacy ISDN to VoIP, not on general-purpose servers.

## EXPOSURE

**Do not expose.** Telephony signalling between gateway elements, with no authentication of its own — it assumes a trusted operator network. Keep it on the signalling network.

## CHECK

```sh
sudo lsof -nP -iTCP:9900 -sTCP:LISTEN
```

## SEE ALSO

- spec — [RFC 4233 — ISDN Q.921-User Adaptation Layer](https://www.rfc-editor.org/rfc/rfc4233.html) (rfc-editor.org)
- 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)

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HTML: <https://whatport.net/9900/> · JSON: <https://whatport.net/9900.json> · full dataset: <https://whatport.net/ports.json>