Port 931 — The Unassigned Port With a Famous Number

Port 931 is unassigned. No official service. No IANA registration. Just an empty slot in the well-known port range (0-1023).

But the number 931 matters in Internet history—just not for this port.

The RFC 931 Story

In January 1985, [RFC 931]¹ was published, defining the Authentication Server Protocol. The protocol provided a way to determine the identity of a user on a particular TCP connection. Given a pair of TCP port numbers, it would return a character string identifying the owner of that connection.

RFC 931 didn't specify that the protocol should run on port 931. It was just assigned RFC number 931 in the document series.

The Authentication Server Protocol was later revised and standardized as RFC 1413 in February 1993², renamed to the Identification Protocol (commonly known as "ident" or "auth"). The service was assigned to port 113, where it still officially resides today.

So RFC 931 became famous. Port 931 stayed empty.

Why Port 931 Is Unassigned

Port 931 falls in the well-known port range (0-1023), also called system ports. These are assigned by IANA and typically require administrative privileges to bind to on Unix-like systems.

Well-known ports are supposed to be carefully managed. Each assignment goes through IETF review. But not every number gets used. Port 931 is one of the gaps.

What Runs Here (If Anything)

Since port 931 is unassigned, you might find:

Nothing—Most systems leave it closed. No service listening. No traffic.

Local services—Some administrators use unassigned ports for internal applications. If you see something on port 931, it's probably organization-specific, not a standard protocol.

Malware—Unassigned ports are sometimes used by malicious software precisely because they're not expected to have traffic. If you see unexpected activity on port 931, investigate.

How to Check What's Listening

On your system, you can check if anything is using port 931:

Linux/macOS:

sudo lsof -i :931
# or
sudo netstat -tulpn | grep :931

Windows:

netstat -ano | findstr :931

If you see output, something is listening. If not, the port is closed.

Why Unassigned Ports Matter

The Internet has 65,535 ports per protocol (TCP and UDP). About 1,024 are reserved as well-known ports. Another ~48,000 are registered ports (1024-49151). The rest are dynamic/ephemeral (49152-65535).

Unassigned ports in the well-known range represent possibility. They're available for future protocols that earn IANA assignment through the standards process. They're also a reminder that not every number needs to be used—some gaps are fine.

Port 931 is one of those gaps. No service claims it. No protocol needs it. But the number itself echoes in RFC history, attached to a protocol that found its home on port 113 instead.

The Confusion

If you search for "port 931," you'll find references to RFC 931 and the Authentication Server Protocol. This creates confusion—people assume the protocol runs on port 931 because of the RFC number.

It doesn't. It never did. RFC 931 defined a protocol. IANA assigned that protocol to port 113. The numbers just happened to be different.

This is normal. RFC numbers and port numbers come from different registries, assigned independently. Sometimes they align. Usually they don't.