Port 75: Any Private Dial Out Service — A Reserved Door from the Modem Era

The Blank Page

Port 75 carries the IANA designation "any private dial out service."¹ No specific protocol. No RFC defining its behavior. Just a label and a permission: use this port for dialing out, however you see fit.

This makes port 75 one of the most interesting kinds of port assignments, not because of what runs on it, but because of what it represents. It is a deliberate gap, a reserved space in the Internet's nervous system left open by design.

What "Any Private Dial Out Service" Means

In the early days of networking, "dial out" meant exactly what it sounds like. A computer, connected to a modem, would literally dial a telephone number to establish a connection with another machine. This was how networks talked to each other before persistent connections became the norm.

Jon Postel, who managed IANA's port assignments from the beginning, reserved several well-known ports for "any private" use:²

• Port 24: any private mail system
• Port 35: any private printer server
• Port 57: any private terminal access
• Port 59: any private file service
• Port 75: any private dial out service
• Port 77: any private RJE service

These weren't assignments to specific protocols. They were permissions. Postel was saying: organizations will need to run their own internal services, and they shouldn't have to register a protocol with IANA to do it. Here are ports they can use.

Port 75 specifically was for outbound modem connections, the act of a machine reaching out through the telephone network to connect to something beyond its local environment.

The Man Who Left the Doors Open

Jon Postel began keeping track of protocol numbers on a scrap of notebook paper in 1969.³ He called himself the "czar of socket numbers" in RFC 433. For nearly 30 years, until his death in October 1998, he personally managed the assignment of every port number, every protocol parameter, every piece of the Internet's addressing infrastructure.

His "any private" port reservations reveal something about how he thought. A less imaginative administrator would have assigned every well-known port to a specific protocol or left them unassigned. Postel instead created a middle category: reserved, but flexible. Claimed, but not constrained. He trusted that people building networks would know what they needed better than he could predict.

The Well-Known Port Range

Port 75 sits in the well-known port range (0-1023), which carries specific meaning.¹ These ports are controlled by IANA and on most operating systems can only be opened by processes running with root or system-level privileges. This restriction exists because well-known ports are where critical services live. You don't want unprivileged software claiming to be your SSH server.

The fact that port 75 occupies this privileged range, despite having no standardized protocol, tells you something about how seriously Postel took the need for private services. He didn't relegate them to the registered range (1024-49151). He gave them the same standing as HTTP, FTP, and SMTP.

Modern Reality

Port 75 sees almost no traffic on the modern Internet.⁴ The dial-out modem era that gave it purpose ended years ago. AOL, the last major dial-up provider in the US, shut down its service in September 2025.⁵

Some niche uses have been observed:

• ConnectWise Automate has used port 75 for heartbeat communications
• Various malware has historically used port 75, though this is true of nearly any port, attackers choose unusual ports precisely because they attract less scrutiny

But for practical purposes, port 75 is quiet.

Checking What Listens on Port 75

If something is listening on port 75 on your system, you should investigate. It's unusual enough to warrant attention.

# Linux
sudo ss -tlnp | grep :75
sudo lsof -i :75

# macOS
sudo lsof -i :75
netstat -an | grep '\.75 '

# Windows
netstat -an | findstr :75

Because port 75 has no standard service, anything listening on it is either a custom internal application or something that deserves scrutiny.

Why Unassigned Ports Matter

The port numbering system has 65,535 possible ports. Only a fraction carry official IANA assignments. The unassigned ports aren't wasted space. They're the reason the system works.

Every custom application, every internal tool, every experimental protocol needs somewhere to listen. The unassigned ports provide that space. And ports like 75, with their "any private" designation, represent the earliest recognition that the Internet would need room for things its designers couldn't foresee.

Port 75 is a monument to a specific kind of engineering wisdom: the willingness to leave space for what you can't yet imagine.