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Port 166 (s-net)

IANA assigns port 166 to s-net: Sirius Systems.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
s-net TCP Sirius Systems Brian Lloyd -
s-net UDP Sirius Systems Brian Lloyd -

Check what is listening on port 166

Linux: is anything listening on TCP 166?
sudo ss -tlnp 'sport = :166'
Linux: UDP 166
sudo ss -ulnp 'sport = :166'
Linux / macOS: which process owns the port
sudo lsof -nP -i :166
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':166 '
Windows (Command Prompt)
netstat -ano | findstr :166
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 166 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 166
From Windows
Test-NetConnection HOST -Port 166
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 166

Block port 166 in the firewall

ufw
sudo ufw deny 166/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 166 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 166 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 166' -Direction Inbound -Protocol TCP -LocalPort 166 -Action Block
ufw
sudo ufw deny 166/udp
iptables
sudo iptables -A INPUT -p udp --dport 166 -j DROP
nftables
sudo nft add rule inet filter input udp dport 166 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 166' -Direction Inbound -Protocol UDP -LocalPort 166 -Action Block

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