Port 2757: CNRP — The Protocol That Arrived at Its Own Funeral

What Port 2757 Does

Port 2757 is assigned to CNRP — the Common Name Resolution Protocol — on both TCP and UDP, as registered with IANA.¹

You have almost certainly never connected to it. Almost no one has.

The Problem CNRP Was Solving

In the late 1990s, URLs were an obstacle. Real people did not think in www.companyname.com. They thought in brand names, product names, plain words. The browser address bar was technically correct but humanly hostile.

CNRP's proposal: build a protocol that maps common names to resources. Type "Boeing" into a resolver. Get back https://www.boeing.com. The protocol, defined in RFC 3367 (August 2002), specified an XML-based request/response system where a client sends a query — a word, a phrase, a trade name — and a server returns matching resources with ranked results.²

It was, essentially, search. But as a protocol rather than a web page.

The Company Behind It

RealNames Corporation was the commercial engine behind common-name resolution. Their product let users type keywords directly into Internet Explorer's address bar — Microsoft distributed the service with the browser. RealNames engineers helped draft the CNRP specification at the IETF.

In May 2002, Microsoft chose not to renew the contract. RealNames shut down.³

RFC 3367 was published three months later, in August 2002. The standard arrived at its own funeral.

Why It Never Recovered

CNRP died for two reasons that compounded each other:

Google. By 2002, search engines had already won the "resolve a common name to a website" problem, and they did it without requiring a new protocol or browser integration. You type a word, you get a link. The URL bar in modern browsers is a search box. CNRP's premise became the default behavior of every browser on Earth.

No ecosystem. With RealNames gone, there were no commercial resolvers to query. A protocol without servers is a specification without a purpose.

Port Assignment Reality

The primary CNRP port specified in RFC 3367 is 1096 — that's the default HTTP transport port the RFC defines for CNRP service.² Port 2757 appears in the IANA registry as a secondary assignment. In practice, neither port carries meaningful traffic today.

How to Check What's Using Port 2757

If you see traffic or a listener on port 2757 on a modern system, it is not CNRP. Check what's actually there:

macOS / Linux:

# Show what process is listening on port 2757
lsof -i :2757

# Or with ss (Linux)
ss -tlnp | grep 2757

Windows:

netstat -ano | findstr :2757

Any listener on this port is using it informally — the registered service is a ghost.

Why Unassigned (and Ghost-Assigned) Ports Matter

The registered port range (1024–49151) exists because services need stable, predictable addresses. Applications register with IANA so that port 2757 means CNRP everywhere, and two different services don't collide.⁴

Port 2757's story is a reminder that the registry is a snapshot of intentions. A port can be assigned to a service that never shipped, or that shipped and failed. The number persists in the registry long after the protocol stops mattering.

The port belongs to CNRP. CNRP belongs to history.