Port 2042: isis — A Name Without a Protocol

What Port 2042 Is

Port 2042 sits in the registered port range — numbers 1024 through 49151, where applications and services request official assignments from IANA to stake a claim in the port namespace.¹

IANA's registry lists port 2042 as "isis" on both TCP and UDP.²

That assignment is almost certainly wrong — or at least, meaningless.

The IS-IS Problem

IS-IS (Intermediate System to Intermediate System) is a link-state routing protocol. It's what large ISPs and backbone networks use to route traffic across their infrastructure, choosing shortest paths through a web of interconnected routers.³

But IS-IS doesn't ride on IP. It operates at layer 2 — the data link layer, below IP entirely. Because it doesn't use IP, it doesn't use TCP or UDP. Because it doesn't use TCP or UDP, it doesn't use port numbers.

Reserving a TCP port for IS-IS is a bit like listing a phone number for a radio tower. The concepts don't connect.

This appears to be a legacy registration with no practical implementation behind it. No IS-IS software sends traffic to TCP port 2042. No IS-IS documentation references it. The name is in the registry, but the protocol it refers to lives in a different layer of the stack entirely.⁴

What You Might Actually Find on Port 2042

In practice, port 2042 has no dominant unofficial use. It appears in port scanner results when:

• Custom or internal applications claim it for development or testing
• Some database tools or monitoring agents use it as an alternative port
• Malware occasionally picks unassigned ports like this to avoid detection

If you see something listening on port 2042, it's running something specific to that machine — not a standard service you can look up.

How to Check What's Listening

On Linux or macOS:

# Show what process is using port 2042
ss -tlnp | grep 2042
# or
lsof -i :2042

On Windows:

netstat -ano | findstr :2042

The output will give you a process ID (PID). Cross-reference that with Task Manager or ps aux | grep <PID> to find the actual application.

Why Registered Ports Still Matter

Even ports with nothing officially assigned serve a purpose. The registered range exists so that applications don't accidentally collide with each other — a web server, a database, and a monitoring agent can all request their own reserved numbers and avoid stepping on each other's traffic.

Port 2042's "isis" entry is a minor oddity in a registry with over 49,000 entries. Most go unnoticed. But it's a good reminder that port registrations are made by humans, humans make mistakes, and the registry is a living document that sometimes preserves historical decisions that no longer reflect reality.