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Port 551 (cybercash)

IANA assigns port 551 to cybercash.

Well-known (system) port, 0-1023 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
cybercash TCP cybercash Donald E Eastlake RFC 1898
cybercash UDP cybercash Donald E Eastlake RFC 1898

Check what is listening on port 551

Linux: is anything listening on TCP 551?
sudo ss -tlnp 'sport = :551'
Linux: UDP 551
sudo ss -ulnp 'sport = :551'
Linux / macOS: which process owns the port
sudo lsof -nP -i :551
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':551 '
Windows (Command Prompt)
netstat -ano | findstr :551
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 551 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 551
From Windows
Test-NetConnection HOST -Port 551
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 551

Block port 551 in the firewall

ufw
sudo ufw deny 551/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 551 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 551 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 551' -Direction Inbound -Protocol TCP -LocalPort 551 -Action Block
ufw
sudo ufw deny 551/udp
iptables
sudo iptables -A INPUT -p udp --dport 551 -j DROP
nftables
sudo nft add rule inet filter input udp dport 551 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 551' -Direction Inbound -Protocol UDP -LocalPort 551 -Action Block

Related ports

549idfp550new-rwho552devshr-nts553pirp

All port numbers · Open port checker
Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.