Port 551 is assigned to CyberCash, a credit card payment protocol from the mid-1990s. Both TCP and UDP are registered. You will almost certainly never see traffic on this port—the protocol has been dead for over two decades.
What CyberCash Was
In 1994, when the web was young and e-commerce was just beginning, a company called CyberCash set out to solve a fundamental problem: how do you safely send credit card information over the Internet?1
They built a system. Consumers installed wallet software. Merchants installed payment software. Credit card numbers were encrypted, transmitted through CyberCash's servers, and processed by banks. It was meant to be the standard way to pay for things online.
In February 1996, they published RFC 1898—the CyberCash Credit Card Protocol Version 0.8.2 They got IANA to assign them port 551. Their stock went public and rose 79% on the first day of trading.3
The future looked bright.
What Happened
Several things went wrong at once.
In January 2000, a teenage Russian hacker cracked their ICVerify application. Days later, the Y2K bug hit their system, causing double charges for credit card payments. Consumer trust evaporated.4
But the deeper problem was simpler: SSL/TLS won. While CyberCash built proprietary wallet software and special protocols, the rest of the Internet converged on HTTPS. Merchants could just put a form on their website. Consumers didn't need to install anything. Payment processors handled the backend.
CyberCash filed for Chapter 11 bankruptcy on March 11, 2001.5 VeriSign bought their assets a few months later. Eventually those assets ended up at PayPal. The protocol itself just... stopped existing.
The Well-Known Ports Range
Port 551 sits in the well-known ports range (0-1023), also called system ports. These are assigned by IANA and typically require privileged access to use. Getting a port number in this range meant something in 1996—it meant your protocol was considered important enough to reserve a spot in the finite space of low port numbers.
The assignment remains. Port 551 is still listed as "cybercash" in the IANA registry. Nobody is using it. Nobody will use it. But it's reserved, forever, for a protocol that no longer exists.
Why This Matters
The Internet is full of ports like this—assigned to protocols that died, companies that went bankrupt, standards that were superseded, experiments that failed. They're archaeological layers. Each one represents someone's attempt to solve a problem, to build something useful, to stake a claim on the future.
Most of them failed. Some were replaced by better solutions. Some were victims of bad timing or bad luck. Some were just wrong about what the Internet would become.
Port 551 is a reminder: getting a port number doesn't guarantee immortality. Having an RFC doesn't mean your protocol will matter in ten years. Even being first doesn't mean you'll win.
Checking Port 551
If you want to see if anything is listening on port 551 on your system:
Linux/Mac:
Windows:
You probably won't find anything. But if you do, it's worth investigating—nobody should be running CyberCash in 2025.
What Runs on Port 551 Today
Nothing, officially. The protocol is dead.
Unofficially, malware sometimes uses assigned-but-abandoned ports because they look legitimate in firewall logs. If you see unexpected traffic on port 551, treat it as suspicious.
The Lesson
Port 551 is a tombstone. It marks the spot where CyberCash tried to build the future of payments and failed. SSL/TLS won. Credit card processing became invisible infrastructure. The special wallet software was forgotten.
But the port number remains—assigned, reserved, waiting for packets that will never arrive. That's how the Internet remembers: not with monuments, but with empty spaces in a registry file.
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