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Port 16666 (vtp)

IANA assigns port 16666 to vtp: Vidder Tunnel Protocol.

Registered (user) port, 1024-49151 UDP

Port 16666 is normally used over UDP; the checker tests TCP only, so a "filtered" result does not mean the UDP service is down.

IANA assignment

Service nameProtoDescriptionAssigneeReference
vtp UDP Vidder Tunnel Protocol Vidder Inc -

Registry dates for this port: registered 2011-10-24.

Check what is listening on port 16666

Linux: UDP 16666
sudo ss -ulnp 'sport = :16666'
Linux / macOS: which process owns the port
sudo lsof -nP -i :16666
Windows (Command Prompt)
netstat -ano | findstr :16666
Windows (PowerShell)
Get-NetUDPEndpoint -LocalPort 16666

Test it from outside

UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 16666

Block port 16666 in the firewall

ufw
sudo ufw deny 16666/udp
iptables
sudo iptables -A INPUT -p udp --dport 16666 -j DROP
nftables
sudo nft add rule inet filter input udp dport 16666 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 16666' -Direction Inbound -Protocol UDP -LocalPort 16666 -Action Block

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Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.