Port 2231: WiMAX ASN Control Plane — A Protocol from the Road Not Taken

Port 2231 is assigned by IANA to wimaxasncp — the WiMAX Access Service Network Control Plane Protocol. It operates on both TCP and UDP, though UDP is typical for the control signaling it carries.

You won't encounter this port on most networks. It belongs to a technology that had its moment and then lost the race.

What It Does

WiMAX (Worldwide Interoperability for Microwave Access) was a wireless broadband standard that competed with 3G and early 4G technologies in the late 2000s and early 2010s. It promised high-speed wireless Internet without the cable — a real alternative to DSL and cable modems, delivered over the air.

Inside a WiMAX network, infrastructure components need to talk to each other: base stations coordinate handoffs as mobile devices move, access gateways manage sessions, and control messages flow between nodes to keep connections alive. That coordination happens over the ASN Control Plane Protocol — wimaxasncp — on port 2231.

Specifically, the protocol handles:

• Mobile station attachment — when a device joins the network
• Session management — maintaining and releasing connections
• Handover coordination — passing a moving device from one base station to another without dropping the connection
• Data path control — establishing and tearing down the routes that carry user traffic

These are the R4, R6, and R8 interfaces defined in the WiMAX Forum Network Architecture specification — the internal plumbing of a WiMAX deployment.¹

The Technology Behind It

The WiMAX Forum published the specification that defines this protocol: Network Architecture, Stage 3: Detailed Protocols and Procedures, Release 1.0.0, March 2007.² Unlike many Internet protocols, wimaxasncp was standardized by an industry consortium rather than through an IETF RFC. The WiMAX Forum drove the standard; the IETF wasn't in the room.

Wireshark has a dissector for the protocol, which tells you something: enough WiMAX was deployed that people needed to analyze its traffic.

The Standards War It Lost

WiMAX had real deployments. Sprint built a nationwide WiMAX network in the United States under the "Clearwire" brand. It had genuine coverage in major cities, real subscribers, and real infrastructure running protocols like the one on port 2231.

Then LTE arrived. Verizon and AT&T pushed LTE hard, equipment manufacturers rallied around it, and the economics of scale followed. By the early 2010s, the outcome was clear. Sprint eventually shut down its WiMAX network in 2015 and migrated to LTE. The WiMAX Forum still exists, but mobile WiMAX is functionally a closed chapter.

Port 2231 sits in the IANA registry as a registered assignment — accurately documenting a protocol that served a real purpose in real networks, now quiet on almost every system you'll find.

Port Range

Port 2231 falls in the registered ports range (1024–49151). These ports are:

• Registered with IANA for specific services
• Not reserved like well-known ports (0–1023), which require elevated privileges to bind
• Not ephemeral — they represent assigned services, not dynamically allocated client ports

Any application can bind a registered port without special permissions, which makes them common for application-layer protocols and server software.

Checking What's Listening

If you see traffic on port 2231 on your network and you're not running WiMAX infrastructure, it's worth investigating. The SANS Internet Storm Center logs occasional scanning activity targeting this port.³

To check what's using port 2231 on a local machine:

Linux / macOS:

ss -tlnp | grep 2231
# or
lsof -i :2231

Windows:

netstat -ano | findstr :2231

If something is listening and you don't recognize it, cross-reference the process ID against your running processes. Unrecognized listeners on obscure registered ports are worth a second look.