Port 51: IMP Logical Address Maintenance — The ARPANET's Address Book

Port 51 is assigned to la-maint, the IMP Logical Address Maintenance protocol. It belongs to the well-known port range (0–1023), ports that are assigned by IANA and historically required root privileges to bind on Unix systems.

Unlike most well-known ports, port 51 carries no modern traffic. The protocol it served, the hardware it ran on, and the network it was built for are all gone. But the story of why it existed reveals something fundamental about how networks stay coherent when things move.

What IMP Logical Address Maintenance Did

The ARPANET connected hosts through Interface Message Processors, specialized minicomputers that functioned as the network's packet-switching nodes. Each host was identified partly by its physical location: which IMP it was connected to, and which port on that IMP.¹

This created a problem. If a host moved to a different IMP, or a different port on the same IMP, its address changed. Every other host on the network that knew the old address now had stale information.

Andrew G. Malis at Bolt Beranek and Newman (BBN) designed a logical addressing layer that decoupled a host's identity from its physical location.² Under this system, hosts received logical names assigned by a central administrator. When a host moved, the administrator updated a database, and the Network Operations Center pushed the changes out to every IMP on the network.

Port 51 carried that maintenance traffic: the address table updates that kept every IMP's view of the network consistent with reality.

The Protocol That Kept the Map Current

The logical addressing system worked through a combination of central authority and distributed updates:

1. A network administrator assigned logical names to hosts at specific IMP ports
2. When a host came online, it sent a Name Declaration Message to its local IMP
3. When address mappings changed, the NOC distributed updates to all IMPs across the network
4. Port 51 carried these synchronization messages

The protocol only ran on BBN's C/30 IMPs. The older Honeywell 516 and Pluribus IMPs didn't have enough memory to hold the new programs and address tables.³

BBN and the IMPs

BBN built the first IMP under a DARPA contract awarded in December 1968. The team was led by Frank Heart, with Robert Kahn handling communications theory, Severo Ornstein on hardware, and Will Crowther, Dave Walden, and Bernie Cosell writing the software.¹

The first IMP was delivered to UCLA on August 30, 1969. On October 29, 1969, the first host-to-host message ("LOGIN," though only "LO" made it before a crash) traveled from UCLA to Stanford Research Institute through these machines.¹

IMPs were the routers before routers existed. They carried the ARPANET from 1969 until DARPA decommissioned the network in 1989. After that, the IMPs were junked, transferred to MILNET, or put in museums. IMP Number One sits on display at UCLA.

The RFC

RFC 878, published in December 1983 by Malis at BBN Communications Corp., specifies the ARPANET 1822L Host Access Protocol, which formalized the logical addressing system that port 51 served.² It obsoleted two earlier versions (RFC 802 and RFC 851), reflecting years of refinement as the ARPANET grew and hosts needed to move more freely between physical locations.

Security

Port 51 carries no modern traffic and has no known active exploits. The protocol and infrastructure it supported have been decommissioned for over three decades.

If you see unexpected traffic on port 51 today, it is not IMP maintenance. It is either misconfigured software or something worth investigating.

How to Check What Is Listening on Port 51

Linux:

sudo ss -tlnp | grep :51
sudo lsof -i :51

macOS:

sudo lsof -i :51

Windows:

netstat -an | findstr :51

On any modern system, nothing should be listening here.

Why Unassigned and Obsolete Ports Matter

Port 51 occupies an interesting position: it is officially assigned, but to a protocol that no longer exists. IANA has never reclaimed it. The reservation persists like a historical marker, recording that this address once meant something to a network that evolved into something larger than its creators imagined.

The well-known port range (0–1023) has 1,024 slots. Some carry the busiest traffic on Earth. Others, like port 51, are quiet monuments to protocols that solved problems so well that their solutions became invisible, absorbed into the assumptions of everything that came after.

Logical addressing, the idea that a host's name should be independent of its physical location, is now so fundamental to networking that it barely registers as an idea. DNS does it. DHCP does it. Every cloud instance that migrates between physical servers does it. Andy Malis and his colleagues at BBN were working out the mechanics of that insight on the ARPANET in the early 1980s, and port 51 carried the updates that made it work.