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Port 21800 (tvpm)

IANA assigns port 21800 to tvpm: TVNC Pro Multiplexing.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
tvpm TCP TVNC Pro Multiplexing Brian Blevins -
tvpm UDP TVNC Pro Multiplexing Brian Blevins -

Check what is listening on port 21800

Linux: is anything listening on TCP 21800?
sudo ss -tlnp 'sport = :21800'
Linux: UDP 21800
sudo ss -ulnp 'sport = :21800'
Linux / macOS: which process owns the port
sudo lsof -nP -i :21800
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':21800 '
Windows (Command Prompt)
netstat -ano | findstr :21800
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 21800 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 21800
From Windows
Test-NetConnection HOST -Port 21800
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 21800

Block port 21800 in the firewall

ufw
sudo ufw deny 21800/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 21800 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 21800 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 21800' -Direction Inbound -Protocol TCP -LocalPort 21800 -Action Block
ufw
sudo ufw deny 21800/udp
iptables
sudo iptables -A INPUT -p udp --dport 21800 -j DROP
nftables
sudo nft add rule inet filter input udp dport 21800 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 21800' -Direction Inbound -Protocol UDP -LocalPort 21800 -Action Block

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