Port 3501: iSoft-P2P — A Registered Port with a Missing Tenant

What Port 3501 Is

Port 3501 sits in the registered port range (1024–49151), the middle tier of the port numbering system. IANA assigns registered ports to specific services on request — they're not free-for-all, but they're not as rigorously standardized as the well-known ports below 1024.

According to the IANA Service Name and Transport Protocol Port Number Registry, port 3501 is assigned to iSoft-P2P on both TCP and UDP, registered to a contact at isoft.com.¹

The Missing Tenant

iSoft was an enterprise software company, active in the early-to-mid 2000s, that built document exchange and healthcare IT systems. Their P2P Agent was designed to let business trading partners exchange documents securely over the Internet — a private, enterprise-grade peer-to-peer layer predating modern API-based integrations.

iSoft was subsequently acquired. The company no longer exists in its original form. The isoft.com contact in the IANA registry is a historical artifact.

This makes port 3501 a ghost registration: officially claimed, practically unclaimed. No software actively ships to use it. No modern documentation describes its protocol. The slot in the registry is occupied, but the occupant left the building years ago.

What That Means in Practice

If you see traffic on port 3501, it is almost certainly not iSoft-P2P. It's more likely:

• A development server or custom application that chose an arbitrary registered port
• Internal tooling configured by a network administrator
• A misconfigured or malicious process using an obscure port on the assumption it won't be monitored

None of these are inherently alarming, but unexpected listeners on any port deserve investigation.

How to Check What's Listening

On Linux/macOS:

# Show what process is listening on port 3501
sudo ss -tlnp sport = :3501

# Or with lsof
sudo lsof -i :3501

On Windows:

netstat -aon | findstr :3501

Then match the PID to a process name:

tasklist | findstr <PID>

Why Ghost Registrations Exist

The registered port range has over 48,000 slots. Many were claimed by companies or projects that no longer exist. IANA's policy historically prioritized assignment over reclamation — once a port is registered, it stays registered unless the holder formally releases it.

This creates a kind of port archaeology: slots that are technically claimed but functionally open, held by software that ships nowhere and companies that exist nowhere. Port 3501 is one of hundreds like it.

For practical purposes, this means the registered range is not a reliable guide to what software actually uses what port. It's a historical ledger as much as an operational one.