Port 391 is officially assigned to the SynOptics SNMP Relay Port—a network management service from a company that no longer exists. It's a well-known port (in the 0-1023 range reserved for official services), but the story behind it is more interesting than the protocol itself.
What Port 391 Was For
Port 391 carried SNMP (Simple Network Management Protocol) relay traffic for SynOptics Communications' device management system. The idea was simple: a central relay service would receive SNMP messages from various network devices and forward them to the appropriate management station.
Think of it as a switchboard operator for network management traffic. Instead of every device talking directly to every management station, they'd route through port 391, which would handle the forwarding.
The port operates on both TCP and UDP, though UDP was the primary transport—SNMP traffic tends to be lightweight status updates where occasional packet loss is acceptable.
The Company That Claimed This Port
SynOptics Communications was founded in 1985 by two engineers from Xerox PARC—the legendary research lab that invented Ethernet, the graphical user interface, and the laser printer. SynOptics didn't invent Ethernet, but they revolutionized how it got deployed.1
Before SynOptics, Ethernet meant thick coaxial cables snaking through office ceilings. SynOptics popularized the Ethernet hub—a box that let you plug in twisted-pair copper cables (the same kind used for phones) and suddenly you had a network. Simpler. Cheaper. Faster to install.
By the early 1990s, SynOptics was the market leader in Ethernet hubs, ahead of rivals like 3Com and Cabletron.1 When you needed to manage hundreds of these devices across a corporate network, you needed a way to monitor them. That's where SNMP came in—and where port 391 got assigned.
What Happened to SynOptics
In 1994, SynOptics merged with Wellfleet Communications (a Massachusetts-based router manufacturer) to form Bay Networks. The name came from their locations: San Francisco Bay Area and Massachusetts Bay.2
The logic was sound. SynOptics made hubs. Wellfleet made routers. Together they could compete with Cisco, which was dominating both markets. The merger created a networking giant worth billions.
But Bay Networks struggled. Internal conflicts between the two engineering cultures. Product lines that didn't integrate well. A competitor (Cisco) that was faster and more focused.
In 1998, Northern Telecom (Nortel) acquired Bay Networks for $9.1 billion.2 Nortel itself collapsed in 2009, one of the largest bankruptcies in Canadian history.
Port 391 outlived all of them.
The Ghost in the Registry
Here's the strange part: port 391 is still officially assigned to "synotics-relay" in the IANA registry.3 The contact listed is Illan Raab, presumably someone who worked at SynOptics in the early 1990s.
SynOptics hasn't existed since 1994. Bay Networks hasn't existed since 1998. Nortel hasn't existed since 2009. But port 391 remains reserved for the SynOptics SNMP Relay Port.
Nobody's using it. The protocol it was designed for is obsolete. Modern SNMP implementations use ports 161 and 162 directly—no relay needed. But the port number persists in the official registry, a tombstone for a company that helped build the Internet's physical layer.
Why This Matters
Port 391 is a reminder that the Internet is an archaeological site. Every port number tells a story about what someone, somewhere, was trying to solve. Some ports carry protocols that power billions of devices. Others, like 391, are fossils—evidence of technologies that mattered once and then didn't.
The well-known port range (0-1023) is finite. There are only 1,024 slots, and they're assigned carefully by IANA through a formal review process.4 Once assigned, they're rarely reclaimed, even when the service dies.
This creates a strange permanence. Port 391 will likely remain "synotics-relay" forever, long after the last person who remembers SynOptics is gone.
How to Check What's Using Port 391
If you want to see if anything is actually listening on port 391 on your system:
Linux/Mac:
Windows:
Chances are you'll find nothing. Port 391 is almost certainly silent on your machine. But if something is there, it's either:
- Legacy network management software that nobody bothered to update
- Malware squatting on an unused port (rare but possible)
- A custom application that picked 391 because it was "available"
Related Ports
SynOptics was assigned a cluster of ports for its network management system:3
- Port 391: SNMP Relay Port
- Port 392: SynOptics Port Broker Port
- Port 412: Trap Convention Port
All three are ghosts now. All three remain in the registry.
The Honest Truth
Port 391 doesn't matter anymore. It's not carrying critical traffic. It's not powering infrastructure. It's not teaching us anything about modern networking.
But it's honest. It shows us that the Internet isn't just the protocols we use today—it's every protocol we've ever used, layered on top of each other, with the old ones quietly fading but never quite disappearing.
Port 391 is a love letter to a company that no longer exists, for a service that nobody needs, in a registry that will probably outlive us all.
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