Port 3675: CallTrax Data Port — A Registered Ghost

What Port 3675 Is

Port 3675 sits in the registered ports range (1024–49151), the middle tier of the port numbering system. IANA officially assigned it the name CallTrax Data Port in January 2003.¹

No RFC was written for it. No open-source implementation exists. The protocol that ran on it was proprietary, belonging to whatever application called itself CallTrax in 2003.

The Registered Ports Range

The port number system has three tiers:

• Well-known ports (0–1023): The foundations — HTTP, SSH, DNS, SMTP. Controlled tightly by IANA, requiring IETF review to assign.
• Registered ports (1024–49151): Where applications stake their claim. A company or developer petitions IANA, pays nothing, and receives a permanent assignment. No ongoing accountability required.
• Dynamic/ephemeral ports (49152–65535): Temporary ports assigned by the OS for outbound connections. No permanent assignments here.

Port 3675 is a registered port, which means IANA logged it into the registry in good faith. Whether the registrant still uses it is another matter.

What CallTrax Was

The IANA registry entry reads: calltrax — CallTrax Data Port — 2003-01.¹

Products currently carrying the "CallTrax" name are modern web-based call tracking services — marketing analytics tools that route phone numbers to measure ad campaign performance. It is unclear whether any of them are related to the 2003 registrant or whether they use port 3675 at all. The name may simply have been reused independently.

The original CallTrax Data Port — whatever protocol it defined, whatever client-server handshake it performed — is effectively undocumented in the public record. It registered an address and then went quiet.

Checking What Is Actually on This Port

If you see traffic on port 3675, it is almost certainly application-specific to whatever is running on your network — not a universal protocol you can look up.

To find what is listening on this port on your system:

Linux/macOS:

# Show processes listening on port 3675
ss -tlnp sport = :3675

# Or with lsof
lsof -i :3675

Windows:

netstat -ano | findstr :3675
# Then look up the PID:
tasklist | findstr <PID>

Scanning a remote host:

nmap -sV -p 3675 <host>

If something is listening, the process name will tell you more than the port number ever could.

Why Unassigned-in-Practice Ports Matter

The registered ports range has over 48,000 slots. Many of them are like port 3675: officially assigned, effectively abandoned, their protocols known only to the engineers who wrote them two decades ago.

This matters for a few reasons:

Security scanning: Port scanners flag open ports against known service lists. A process squatting on port 3675 will not match any known signature, which makes it worth investigating — not because 3675 is dangerous, but because unexpected listeners on any port deserve scrutiny.

Firewall rules: Blocking "all registered ports" is not a useful policy. Understanding that most registered ports are quiet and application-specific helps you write rules that reflect your actual traffic.

Port conflicts: If you build an application and want a permanent port, you apply to IANA. But many developers simply pick a number from the registered range without checking — and occasionally land on a slot already taken by a ghost like CallTrax Data Port.