Port 375 sits in the well-known port range (0-1023), reserved for system services and protocols that shaped the early Internet. It was assigned to HASSLE—Hierarchical Access System for Sequence Libraries in Europe—a protocol that let scientists access DNA and protein sequence databases remotely.1
What HASSLE Was
In 1994, Reinhard Doelz published HASSLE in the journal Computer Applications in the Biosciences (now Bioinformatics).1 It was an application-independent, user-transparent access tool for molecular biology databases. Think of it as FTP for genetic sequences—a way to query and retrieve biological data across institutional and national boundaries.
At the time, this mattered. Bioinformatics was nascent. The Human Genome Project was underway. Scientists needed standardized ways to share sequence data across networks. HASSLE was one attempt to solve that problem.
The protocol was assigned port 375 for both TCP and UDP, documented in RFC 1700 (Assigned Numbers) in October 1994.2 Doelz, working at the Biozentrum of the University of Basel in Switzerland, was listed as the contact for the assignment.
What Happened to It
HASSLE is dead. The protocol isn't used anymore. Modern bioinformatics uses HTTP APIs, REST endpoints, and specialized tools like BLAST. The scientific databases that HASSLE connected to—EMBL, GenBank, SwissProt—are now accessed through web interfaces and standardized protocols like NCBI's Entrez system.
But the port assignment remains. IANA still lists port 375 as assigned to "hassle" with Reinhard Doelz as the contact.3 This is common. Port assignments rarely get revoked, even when the services they were assigned for disappear.
The Well-Known Port Range
Port 375 sits in the well-known port range (0-1023), which requires root/administrator privileges to bind to on Unix-like systems. This range was intended for standardized services assigned by IANA. Most of these ports were claimed in the 1970s through 1990s, during the Internet's formative years.
HASSLE got in late—1994 was near the end of the era when individual researchers could request well-known port assignments for specialized protocols. After RFC 1700, IANA moved away from static RFC-based port lists to a living registry, and the criteria for well-known port assignments became much stricter.
Why Unassigned Ports Matter
Port 375 isn't truly unassigned—it has an official assignment—but it's functionally unused. This matters because it illustrates a truth about the port number system: it's a historical record as much as it's a functional registry.
Thousands of ports have official assignments for services that no longer exist. They're fossils of protocols that mattered once, in contexts we've moved beyond. Port 375 mattered when scientists needed a standardized way to access sequence databases remotely. Now they don't. The protocol died. The assignment remains.
How to Check What's Listening
If you want to see what's actually using port 375 on your system:
On Linux/macOS:
On Windows:
You'll almost certainly find nothing. HASSLE is extinct. If you do find something listening on port 375, it's either:
- A modern application repurposing an old assignment
- A research system running legacy bioinformatics tools
- Something else entirely that chose this port because it's officially assigned but practically unused
The Lesson
Port 375 is a reminder that the Internet carries its history in its infrastructure. Every assigned port represents a moment when someone thought a protocol mattered enough to reserve a number for it. Most of those protocols are gone now. The ports remain, quiet monuments to things that used to be essential.
HASSLE mattered in 1994. Scientists needed it. Now they don't. But port 375 still belongs to HASSLE, three decades later, waiting for traffic that will never come.
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