Port 61: NI MAIL — The Revoked Seat

Port 61 is a well-known port with no active assignment. It sits in the system port range (0 through 1023), the range controlled by IANA and typically reserved for foundational Internet services. But port 61 has no service. Not anymore.

It used to.

What Was NI MAIL

Port 61 was originally assigned to NI MAIL, a mail protocol associated with Network Innovations¹. Its companion, NI FTP, lived on port 47. Both were registered by Steve Kille, then a researcher at University College London, as part of early work on networked messaging systems.

The "NI" prefix stood for the protocol suite Kille was developing. These were the 1980s, and the question of how email should work on interconnected networks was genuinely unsettled. SMTP existed (port 25 had been assigned in 1982), but the OSI world was pushing X.400 as the "proper" standard for electronic messaging, and Kille was one of the people building bridges between the two worlds².

Kille went on to co-author LDAP (Lightweight Directory Access Protocol), write over forty RFCs, and found Isode Limited, which still builds messaging and directory products today³. Port 61 was one of his earliest marks on the Internet's infrastructure.

The De-Assignment

On May 18, 2017, IANA formally removed the NI MAIL assignment from port 61, marking it as Reserved⁴. This is uncommon. Most well-known ports keep their assignments forever, even when the protocol they carry has been dead for decades. Port 61 didn't get that courtesy.

Under RFC 6335, IANA can de-assign a port only after establishing that the service is genuinely no longer in use, and the process requires involvement of an IESG-appointed expert⁵. The fact that this process completed tells you something: NI MAIL didn't just fall out of fashion. It was formally declared gone.

The Range

Port 61 belongs to the System Ports range (0 through 1023). These ports are the most controlled addresses in networking. On most operating systems, only privileged processes can bind to them. They were intended for the Internet's most essential services: HTTP, SMTP, DNS, SSH.

Having a system port assigned was, in the early days, a mark of significance. Losing one is a quiet kind of erasure.

Checking Port 61

To see if anything is listening on port 61 on your system:

# macOS / Linux
sudo lsof -i :61

# Windows
netstat -an | findstr :61

# Scan a remote host
nmap -p 61 target-host

If something is listening on port 61 today, it is not running NI MAIL. It is either a custom application, a misconfiguration, or something worth investigating.

Why Unassigned Ports Matter

There are 1,024 well-known ports. Not all of them carry active services. Some, like port 61, once had assignments that were later revoked. Others were never assigned at all.

These empty ports are not wasted space. They are available capacity. Every unassigned well-known port is a slot that could be given to the next protocol that earns its place in the Internet's foundation. Under IANA policy, de-assigned ports like 61 will not be reassigned until all truly unassigned ports in the range have been used first⁵. The Internet protects against confusion by keeping retired names off the field for as long as possible.

Port 61 is a reminder that the port table is not just a technical registry. It is a record of what the Internet tried, what worked, and what was eventually let go.