Port 720 sits in the well-known port range (0-1023), the most controlled section of the port numbering system. These ports are supposed to be officially assigned by IANA through rigorous IETF review processes. Port 720 never received that blessing.
What "Unassigned" Means
When a port in the well-known range is unassigned, it means:
- No official service — IANA hasn't designated what should run here
- No RFC — No formal specification defines its use
- Still reserved — Being in the 0-1023 range means it can't be used without root/administrator privileges on Unix-like systems
The well-known range exists precisely to prevent chaos—to ensure critical services have predictable, stable port numbers. Port 720 is a gap in that structure.
The SMQP Story
Despite being unassigned, port 720 appears in various port databases as the default port for SMQP (Simple Message Queue Protocol)—a lightweight messaging protocol designed for distributed systems.1
SMQP was proposed in an Internet-Draft by Jonas Tegen, going through multiple revisions between the early 2000s. The protocol was designed to be simpler than AMQP or other message queuing systems, with the server listening on TCP port 720 by default and clients typically using port 721.2
But here's the thing: SMQP never became an official IETF standard. The Internet-Draft expired. The protocol never went through the formal IANA port assignment process required for well-known ports. Yet it claimed port 720 anyway, and that claim lives on in port databases and documentation scattered across the Internet.
This is what happens when someone writes "the server listens on port 720 by default" in a specification without actually registering it.
Why This Port Matters
Unassigned well-known ports are uncommon. Most ports in the 0-1023 range were claimed decades ago for protocols that may be obsolete but remain officially assigned. Port 720's unassigned status makes it an anomaly.
It represents a kind of institutional gap—a space where someone tried to build something, documented it, maybe even deployed it, but never formalized it. The protocol exists in expired drafts and old documentation, but not in the official registry.
What Runs Here
On most systems, nothing. Port 720 sits unused unless:
- Someone is running a custom implementation of SMQP
- A local service has been configured to use this port
- An application claimed it without knowing the history
To check what's listening on port 720:
If you find something running here, it's either a custom application or someone who read that old SMQP draft and decided to implement it.
The Port Range Context
Port 720 belongs to the well-known port range:
- Ports 0-1023 — System/well-known ports, assigned by IANA
- Require elevated privileges to bind on Unix-like systems
- Reserved for standardized services with formal specifications
- Changes require IETF Review or IESG Approval per RFC 63353
For comparison, unassigned ports in the registered range (1024-49151) are common and easier to claim. But in the well-known range, unassigned ports are rare gaps—either waiting for something important enough to deserve the privilege, or simply forgotten.
Why Unassigned Ports Matter
Every unassigned port is potential space. In the well-known range, that space is valuable—limited to just 1024 slots, most already claimed by protocols that defined the early Internet.
Port 720's unassigned status means:
- Future availability — It could be officially assigned if a protocol important enough comes along
- No conflicts — No globally recognized service means local use is safe (within your network)
- Historical artifact — The SMQP claim shows how informal use happens even in controlled spaces
The IANA registry is not just a technical database. It's a kind of archaeological record—showing what got formalized, what didn't, and what tried to exist in the gaps between.
Port 720 lives in one of those gaps.
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