Port 3125: A13-AN Interface — A Relic of the CDMA Era

What This Port Is

Port 3125 is not unassigned — it just looks that way because the service that claimed it has faded from common use.

IANA formally assigned port 3125 (both TCP and UDP) to the A13-AN Interface, a signaling protocol from the CDMA2000 mobile network standard. The assignee on record is Douglas Knisely, an engineer involved in CDMA2000 protocol development.¹

You're unlikely to encounter this port in the wild unless you're working with legacy CDMA infrastructure.

What A13-AN Actually Is

CDMA2000 was the mobile data standard used by carriers like Verizon and Sprint in the United States through the 2000s and 2010s — the technology behind 1xEV-DO, the high-speed data layer that preceded LTE.

Within that architecture, mobile devices could hand off between access network nodes (essentially base station controllers) while maintaining active data sessions. The A13 interface was the signaling channel between two access networks during this handoff process. When a device moved from one AN to another, the source and target ANs exchanged session information over A13 — things like authentication state, session parameters, and QoS configuration.

The message flow was straightforward:

• A13-Session Information Request: Target AN asks source AN for session data
• A13-Session Information Confirm/Reject: Source AN responds
• A13-Session Information Response: Session state transferred

Port 3125 was the designated endpoint for this exchange.²

The Registered Port Range

Port 3125 sits in the registered ports range (1024–49151). This range is different from the well-known ports (0–1023), which require root/administrator privileges to bind and carry the protocols that define the Internet's core infrastructure.

Registered ports are claimed by applications and vendors through IANA, but the claim doesn't guarantee enforcement. Any process on your system can bind to port 3125 — the registration is a coordination mechanism, not a lock. The purpose is to reduce accidental conflicts, not to police usage.³

Is This Port in Use Today?

CDMA2000 networks have been largely decommissioned. Verizon shut down its CDMA network at the end of 2022. Sprint's was absorbed into T-Mobile's LTE/5G infrastructure. The A13-AN Interface, and the handoff signaling it carried, no longer has a network to run on.

If you see something listening on port 3125 on a modern system, it's not A13-AN. It's an application that chose this port because it was conveniently unoccupied.

Checking What's on This Port

To see if anything is currently listening on port 3125:

macOS / Linux:

# Show what process is listening on port 3125
sudo lsof -i :3125

# Or with ss (Linux):
ss -tlnp | grep 3125

Windows:

netstat -ano | findstr :3125

If something appears, the process name and PID will tell you what claimed it. Cross-reference the PID with Task Manager or ps aux to identify the application.