Port 728: Unassigned — A number without a protocol

Port 728 is officially unassigned. It belongs to the well-known port range (0-1023), the most restricted address space on the Internet, but no protocol has ever claimed it.

What "Unassigned" Means

The Internet Assigned Numbers Authority (IANA) maintains the official registry of port assignments. Port 728 appears in that registry as part of an unassigned block (ports 717-728) for both TCP and UDP.¹

This doesn't mean the port is broken or unavailable. It means:

• No standard protocol uses this port — Unlike port 80 (HTTP) or port 443 (HTTPS), there's no RFC defining what should run here
• Applications can still use it — Any program can listen on port 728, but it would be non-standard
• It's waiting — IANA keeps these ports in reserve for future protocols that may need well-known port assignments

Why Keep Ports Unassigned?

The well-known port range (0-1023) is finite. Only 1,024 numbers exist in this space, and many were claimed decades ago when the Internet was young.

IANA doesn't assign these ports casually. A protocol needs to demonstrate:

• Widespread deployment or intention to deploy broadly
• A legitimate need for a well-known port (some protocols work fine with dynamic port assignment)
• Proper documentation in the form of an RFC or similar standard

Port 728 sits in reserve. Maybe someday a protocol will need it. Maybe it will remain empty forever. The well-known range has many such gaps—numbers that seemed necessary to reserve in the 1980s but were never claimed.

What Might Be Listening on Port 728

Just because a port is unassigned doesn't mean nothing uses it. You might find:

• Custom applications — Developers sometimes choose arbitrary ports for internal services
• Malware — Unassigned ports are sometimes exploited because they're unexpected
• Nothing — Most likely, nothing is listening here at all

To check what's running on port 728 on your system:

# On Linux/Mac
sudo lsof -i :728
sudo netstat -tulpn | grep :728

# On Windows
netstat -ano | findstr :728

If something appears, it's non-standard. Investigate what it is.

The Geography of Port Space

The port number system has three ranges:

• Well-known ports (0-1023) — Where port 728 lives. Restricted, assigned by IANA, require root/administrator privileges to bind
• Registered ports (1024-49151) — Available for registration with IANA, used by many common applications
• Dynamic/private ports (49152-65535) — Free for temporary use, assigned automatically by operating systems

Port 728 sits in the most exclusive neighborhood. It's been held in reserve for decades. And it remains unclaimed—a number in the registry with no story to tell. Yet.