Port 738 is unassigned. Despite sitting in the well-known ports range—the Internet's most privileged address space—it has never been claimed by any protocol.
The Well-Known Ports Range
Ports 0-1023 are called the well-known ports. This is the Internet's prime real estate. These ports are managed by IANA (Internet Assigned Numbers Authority) and are meant for fundamental, widely-used protocols.
This is where you find:
- Port 80: HTTP
- Port 443: HTTPS
- Port 22: SSH
- Port 25: SMTP
- Port 53: DNS
Getting a well-known port assignment requires going through IANA's formal process. The protocol needs to be documented, typically in an RFC. It needs to be significant enough to warrant permanent allocation.
Port 738's Story: Nothing to Tell
Port 738 has no story. It was never assigned. No protocol was ever documented for it. No RFC claims it. No service runs on it by convention.
The range from ports 732-740 sits largely empty—a cluster of unassigned addresses in what's supposed to be the most used part of the port space.1
This isn't unusual. Even in the well-known range, many ports remain unassigned. Not every number needs to be used.
Why Keep Ports Unassigned?
You might wonder why IANA doesn't just assign every port. The answer: scarcity creates meaning.
If every port were allocated, the assignments would become meaningless. An unassigned port is available for future protocols we haven't imagined yet. The Internet was designed with room to grow.
Keeping ports unassigned also helps with security. When you scan a network and find port 738 open, it immediately raises a question: "What's running there?" There's no legitimate default service. If something is listening, it's either:
- A custom application
- Malware
- A misconfigured service
- Someone testing
Checking What's on Port 738
Even though port 738 is officially unassigned, something could be running on it. Here's how to check:
On Linux or macOS:
On Windows:
Using nmap to scan remotely:
If nothing is returned, the port is closed. If something appears, you've found a custom service using unassigned space.
The Honest Reality
Most unassigned well-known ports will stay unassigned forever. The protocols that needed ports in this range were mostly created in the 1970s, 80s, and 90s. The foundational layer of the Internet is largely settled.
Modern protocols—the ones being designed today—use the registered ports range (1024-49151) or dynamic ports (49152-65535). They don't need the prestige of a well-known port number.
Port 738 is a door that was built but never opened. And that's fine. Not every space needs to be filled.
Related Ports
- Ports 732-740 — A cluster of unassigned ports in the same range
- Port 1 — Officially assigned to TCPMUX, but rarely used in practice
- Port 512-514 — BSD rlogin/rsh services, once common, now largely abandoned
Frequently Asked Questions About Port 738
- GRC Port Authority - Port 7382
- SpeedGuide Port Database - Port 7381
- IANA Service Name and Transport Protocol Port Number Registry
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