Port 10 is unassigned. It has no official protocol, no registered service, no RFC defining its purpose. It sits in the well-known port range (0 through 1023), between port 9 (Discard) and port 11 (Active Users), and it has been empty since the first port assignments were made in 1972.
This is not an oversight. It is an artifact of how the Internet's ancestors counted.
Why Port 10 Was Never Assigned
In 1972, Jon Postel published RFC 349, "Proposed Standard Socket Numbers," the first formal attempt to assign specific numbers to specific network services.1 The assignments were:
- Socket 1: Telnet
- Socket 3: File Transfer
- Socket 5: Remote Job Entry
- Socket 7: Echo
- Socket 9: Discard
Notice the pattern. Every assigned number is odd. This was not arbitrary.
The ARPANET ran on the Network Control Protocol (NCP), which used simplex connections, meaning data could only flow in one direction per connection.2 To have a two-way conversation, you needed two connections: one for sending, one for receiving. NCP encoded this directly into the socket numbers. Odd numbers were send sockets. Even numbers were receive sockets. When you connected to Telnet on socket 1, the protocol would also open socket 2 as the paired receive channel.
Even-numbered sockets were not services. They were the return paths. Socket 10 was the implicit receive pair for whatever might have lived on socket 11.
On January 1, 1983, the ARPANET switched from NCP to TCP/IP in what is known as "flag day."3 TCP provides full-duplex communication over a single port, making the even/odd pairing obsolete overnight. The odd-numbered ports already had names and histories. The even-numbered ports had been infrastructure, not identity. Most of them, including port 10, simply carried forward as unassigned.
The Well-Known Port Range
Port 10 belongs to the System Ports range (0 through 1023), also called well-known ports. These are controlled by IANA (the Internet Assigned Numbers Authority) and require IETF Review or IESG Approval for assignment.4 On most operating systems, binding to a port in this range requires root or administrator privileges.
Being unassigned in this range does not mean port 10 is free for anyone to use casually. It means IANA has never received and approved a request to assign it. The well-known range carries weight. Services assigned here are expected to be fundamental to the functioning of the Internet. Port 10 simply never had a protocol that needed to live here.
Known Unofficial Uses
Port 10 has no widely recognized unofficial uses. The SpeedGuide ports database notes one minor observation: some AT&T 5268ac routers have been observed listening on port 10/TCP.5 This appears to be a vendor-specific management interface, not a community convention.
No major trojans or malware families are documented as using port 10 as a default communication channel.
How to Check What Is Listening on Port 10
On most systems, you can check whether anything is bound to port 10:
Linux:
macOS:
Windows:
If something is listening on port 10 and you did not configure it, investigate. Unassigned ports that are unexpectedly open deserve attention precisely because no standard service should be there.
Why Unassigned Ports Matter
The Internet's port system has 65,535 numbers, and the majority of them are unassigned. This is by design. The system works because it is sparse. Not every number needs a name. The unassigned ports serve as available space, room for the next protocol, the next service, the next idea that someone has not had yet.
Port 10 has been waiting since 1972. It may wait forever. There is nothing wrong with that. Not every door needs something behind it. Some doors exist so the hallway makes sense.
Frequently Asked Questions
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