Port 551CyberCash

Historical CyberCash electronic-payment traffic.

transport: tcp · udp
in transit: encrypted
assignment: official
risk: 1/10
lookups: 0

also known as CyberCash payment protocol

Technical Details

what runs on :551

CyberCash is registered on port :551 over tcp and udp to Donald E Eastlake. We don't have a technical write-up for it beyond the IANA registry entry.

In nmap's internet-wide scan data this port is found open rarely. What's listening is often not the registered service, so check before you assume.

find what's listening on :551

this machine

# linux
sudo ss -tulpn 'sport = :551'

# macos / bsd
sudo lsof -nP -i :551

# windows (then: tasklist /FI "PID eq <pid>")
netstat -ano | findstr :551

another host

nmap -sV -p T:551 <host>
sudo nmap -sU -sV -p U:551 <host>

run it? the handshake, the implementations you meet in the wild and the flags that matter come from the engineers who do.

add technical details

Security Information

exposure of :551

risk score

1/ 10safe

routine exposure. this port is rarely the way in on its own — keep it patched and logged and move on.

web services averages 3.2 across 185 ports — this one sits 2.2 below.

in transit

encrypted

payloads are protected on the wire

assignment

official

registered with iana for this service — scanners fingerprint it by number

reachable over

tcp · udp

udp is connectionless — source addresses are trivially spoofed and it is a reflection favourite

security overview

no security notes recorded for :551 yet

known exploits, hardening advice, what a compromise looks like — suggest an edit if you know its exposure.

Record

iana registry entry

port: :551
service: CyberCash
transport: TCPUDP
encryption: tls / encrypted
assignment: iana registered
risk: 1/10safe
category: Web Services
usage: This assignment identifies the CyberCash electronic-payment service, a historical Internet payment system. IANA registered the name to Donald E. Eastlake; contemporary deployments are expected to be uncommon.
lookups: 0rank 993
added: 2026-08-23
updated: 2026-08-23
references: IANA service name registry — port 551
RFC 1898

Something wrong?

This record is maintained by the engineers who use port 551. Every field on it is editable — change what's wrong, say how you know, and a reviewer publishes it, usually within a minute. Last touched 2026-08-23.

01edit any field on a page of its own
02a model checks it for accuracy and spam
03clean and confident goes live immediately

edit this record →back to the registry

Related Ports

all 185 in web services →

the 8 most looked-up other ports in web services — 185 ports carry that label.

port	service	proto	category	risk	views
:8888	Sun Answerbook & Alt HTTP	TCP	Web Services	caution	123.9k
:8000	Intel Remote Desktop / Alternate HTTP Port	TCP	Web Services	safe	84.3k
:8888	NewsEDGE	TCPUDP	Web Services	caution	83.3k
:8888	HTTP Alternative Port	TCP	Web Services	caution	76.5k
:8888	GNUmp3d Streaming HTTP	TCP	Web Services	caution	76.3k
:9080	Groove RPC	TCPUDP	Web Services	caution	70.8k
:8881	Atlasz Secure Server	TCP	Web Services	caution	67.6k
:80	HTTP	TCPUDP	Web Services	caution	67.3k

risk mix of the 8 listed

safe13%
caution88%

0 of 8 encrypted

Port :551CyberCash

Technical Details

Security Information

Related Ports

Port 551CyberCash