Port 36: Unassigned — The Empty Door

Port 36 has no assigned protocol. No RFC defines it. No service claims it. It sits in the well-known port range (0-1023) and has been marked "Unassigned" in every version of the IANA Service Name and Transport Protocol Port Number Registry ever published.¹

What "Unassigned" Means

The Internet's port numbering system divides 65,535 ports into three ranges:

• Well-Known Ports (0-1023): Controlled by IANA. On most operating systems, only root or system processes can bind to these ports. Assignment requires IETF Review or IESG Approval.²
• Registered Ports (1024-49151): Available for registration by application developers.
• Dynamic/Ephemeral Ports (49152-65535): Never assigned. Used by operating systems for temporary outbound connections.

Port 36 falls in the first range. It carries the authority of a well-known port but has never been given a purpose. Approximately 24% of well-known ports share this status.²

Its Neighbors

Port 36 sits in an interesting stretch of the registry:

Port 37, its immediate neighbor, has carried time synchronization traffic since 1983.³ Port 35 was reserved for private printer servers. Port 36 sits between them, unclaimed.

No Known Unofficial Uses

Some unassigned ports develop unofficial reputations. Port 8080 became the de facto HTTP alternative long before any formal recognition. Port 36 has no such story. It does not appear in major trojan port databases. It is not commonly associated with any application, game server, or development tool.

It is, by all available evidence, genuinely unused.

Why the Gaps Exist

Port numbers were never assigned sequentially. They were handed out on demand, one at a time, as engineers brought new protocols to the IETF and requested a number. Jon Postel and Joyce Reynolds maintained the Assigned Numbers documents at the Information Sciences Institute, recording each new claim.⁴

Some numbers were simply never requested. No one walked up to the registry and said, "I need port 36." The increasingly strict requirements for well-known port assignment, documented in RFC 6335, make it unlikely anyone will.² A request for a system port must now demonstrate why a registered port or dynamic port would be unsuitable.

These gaps are not errors. They are the natural result of a system designed for longevity. Not every door needs to be opened.

How to Check What Is Listening on Port 36

If you see traffic on port 36, something unexpected is happening. Here is how to investigate:

Linux/macOS:

# Check if anything is listening on port 36
sudo lsof -i :36

# Check with ss (Linux)
sudo ss -tlnp | grep :36

# Check with netstat
sudo netstat -tlnp | grep :36

Windows:

# Check listening ports
netstat -an | findstr :36

Any process bound to port 36 deserves scrutiny. There is no legitimate, well-known service that uses it.