Port 1731 is registered to MS ICCP — Microsoft's Audio Call Control Protocol — on both TCP and UDP. It was one of the ports used by Microsoft NetMeeting, the video and voice collaboration software that shipped with Windows from 1996 onward.
Before Zoom. Before Skype. Before any of it was expected to just work, NetMeeting was what you used when you wanted to have a real-time voice or video call over the Internet. It was clunky, it required walking the other person through firewall settings, and it was genuinely remarkable that it worked at all on a 56k modem.
What MS ICCP Did
ICCP stands for Audio Call Control Protocol. Port 1731 was the control channel — the port that negotiated whether an audio session could be established before any actual audio traveled anywhere.
NetMeeting worked with a cluster of ports rather than a single one:
- Port 1730 — NetMeeting itself (H.323 gatekeeper discovery)
- Port 1731 — Audio Call Control (this port)
- Port 1732 — Audio Call Control alternate
Think of 1731 as the one that asked "are you ready to talk?" before the call began. The actual audio traveled separately, over dynamically assigned UDP ports in the high range (1024–65535), which made network administrators miserable because you couldn't just open one port and be done with it.
Who Was Using It
NetMeeting 2.0 through 3.01 used port 1731 for audio call setup. It also interoperated with H.323, the ITU standard for multimedia communications over packet-switched networks — the same standard that underpinned early Internet telephony and video conferencing broadly.
The OKWin application software also used the port range 1729–1735, overlapping with this cluster.
Where It Stands Now
Microsoft NetMeeting was deprecated with Windows Vista and removed entirely in Windows 7. Port 1731 remains registered with IANA under the MS ICCP name, but nothing actively uses it in modern software. It's a registered port that outlived its application.
If you see traffic on port 1731 today, it's worth investigating. The port has appeared in lists of ports associated with older malware, though that's true of many high-numbered ports that once belonged to legitimate applications.
What's Listening on Your Machine
To check whether anything is using port 1731 locally:
macOS / Linux:
Windows (Command Prompt):
If nothing is listening, the port is closed and irrelevant to your system. If something is, the process ID will tell you what.
What Range This Port Belongs To
Port 1731 is a registered port — the range from 1024 to 49151. These ports are assigned by IANA to specific applications and protocols, but unlike the well-known ports below 1024, they don't require administrator privileges to open. Any application can bind to a registered port. IANA assignment means the port has an official designation, not that the software is still actively used.
The port below it (1730) and above it (1732) are also part of the same NetMeeting cluster, which is typical: applications that need multiple channels often claim a contiguous block of ports rather than scattered ones.
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