Port 1655 is officially registered to dec-mbadmin—the administration service for Digital Equipment Corporation's MAILbus 400 messaging system.1 Both TCP and UDP protocols were assigned this port, though the service itself has been obsolete for decades.
This is a registered port (range 1024-49151), meaning it was formally assigned by IANA to a specific organization for a specific purpose. That organization no longer exists. That purpose no longer matters.
What MAILbus 400 Was
In the late 1980s and early 1990s, Digital Equipment Corporation (DEC) was a computing giant. They built the MAILbus 400 product family—an enterprise messaging system based on the X.400 standard.2
X.400 was supposed to be the future of email. It was an international standard from CCITT (now ITU-T), designed for mission-critical messaging in enterprise and government environments. Unlike SMTP, which grew organically from the early Internet, X.400 was architecturally designed, formally specified, and internationally ratified.
MAILbus 400 included:
- A Message Transfer Agent (MTA) conforming to 1988 CCITT X.400 standards
- Gateways to SMTP, MHS, ccMail, and MS-Mail
- Support for OpenVMS VAX and OSF/1 Alpha AXP platforms
- Tools designed for 24/7 operation in enterprise environments
The dec-mbadmin service on port 1655 handled administrative functions for this messaging infrastructure. Port 1656 (dec-mbadmin-h) was its companion port for related administration tasks.3
What Happened
X.400 lost. SMTP won.
The reasons are familiar to anyone who's watched standards battles: SMTP was simpler, already deployed, good enough, and free. X.400 was complex, expensive, required specialized infrastructure, and solved problems most people didn't have.
DEC itself didn't survive either. Compaq acquired DEC in 1998. HP acquired Compaq in 2002. MAILbus 400 disappeared somewhere in those mergers, a casualty of consolidation and changing technology.
By the mid-2000s, X.400 systems were legacy infrastructure kept alive only where regulatory requirements or institutional inertia demanded it. Today, they're almost entirely gone.
Why This Port Still Exists
IANA doesn't delete port registrations just because the service died. Port 1655 remains officially assigned to dec-mbadmin, a service that hasn't run anywhere in probably 20 years.
This is common in the registered ports range. Companies register ports for products that later fail. Standards bodies reserve ports for protocols that never get adopted. The registry becomes an archaeological record—layer upon layer of things that were supposed to matter.
Port 1655 is unlikely to cause conflicts. Modern systems don't use it. You won't accidentally connect to a MAILbus admin service. But if you scan a network and see something listening here, you're either looking at:
- A very old system that someone forgot to decommission
- A modern application that chose this port specifically because nothing else uses it
- Malware using an obscure port to avoid detection4
Checking What's Listening
On Linux or macOS:
On Windows:
If something is listening on port 1655, it's almost certainly not dec-mbadmin.
The Pattern
Port 1655 represents something larger: the residue of corporate competition from an era when networking protocols were battlegrounds.
DEC registered dozens of ports. So did IBM, HP, Sun, SGI, and every other company that mattered in the 1980s and 90s. Most of those companies are gone now, absorbed into other companies or simply defunct. Most of those protocols are forgotten.
But the port numbers remain. They're cheap to maintain and impossible to reclaim. So the IANA registry fills with ghosts—services that nobody runs, products that nobody remembers, companies that nobody mourns.
Port 1655 is one of those ghosts. The difference between this and port 443 isn't technical excellence. It's that HTTPS won and X.400 lost. History is written by the protocols that survive.
Frequently Asked Questions About Port 1655
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