What This Port Is
Port 3511 sits in the registered ports range (1024–49151) — the middle tier of the port numbering system, between the well-known ports (0–1023) reserved for foundational services and the ephemeral ports (49152–65535) used for temporary connections.
Registered ports are assigned by IANA upon request, theoretically for services that have a reason to exist at a stable, predictable address. In May 2002, port 3511 received such an assignment: WebMail/2, a webmail interface for IBM's OS/2 operating system, created by developer Dimitris Michelinakis.1
What WebMail/2 Was
WebMail/2 was a CGI-based web interface that let OS/2 users read and send email through a browser. It was written in Rexx — OS/2's scripting language — and could connect to OS/2 mail servers like Weasel, InetMail, and IPS.2
OS/2 was IBM's operating system of the early 1990s, a serious contender against Windows that found its strongest following in enterprise environments, particularly banking. By 2002, OS/2 was already fading. WebMail/2 was built for a platform in decline, and the port was registered near the end of that era.
Today, OS/2 exists as a niche hobby system with a small dedicated community. WebMail/2 still has a website. Port 3511, once registered, cannot simply be un-registered — the IANA registry doesn't work that way.
What This Means in Practice
On any modern network, port 3511 is functionally unassigned. No mainstream software listens on it by default. If you see traffic on port 3511, it's almost certainly custom application traffic, a misconfiguration, or scanner noise.
The registered ports range is full of entries like this — software that mattered to someone once, formally staked its claim on a number, and then quietly stopped running on most machines while the registration persisted.
How to Check What's Listening
If port 3511 is active on your machine and you want to know why:
On Linux or macOS:
On Windows:
The process ID from these commands will tell you exactly what's using the port.
Why Unassigned Ports Matter
The registered ports range exists to prevent collisions. When a developer registers a port, they're staking a claim so that two different applications don't independently choose the same number and end up fighting for the same address.
The system works well for widely-adopted services. It works less well when software fades — the reservation stays, but the port sits idle on most of the world's machines. Port 3511 is one of thousands in this situation: officially spoken for, practically available.
Frequently Asked Questions
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