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Port 1821 (donnyworld)

IANA assigns port 1821 to donnyworld.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
donnyworld TCP donnyworld Don Oliver -
donnyworld UDP donnyworld Don Oliver -

Check what is listening on port 1821

Linux: is anything listening on TCP 1821?
sudo ss -tlnp 'sport = :1821'
Linux: UDP 1821
sudo ss -ulnp 'sport = :1821'
Linux / macOS: which process owns the port
sudo lsof -nP -i :1821
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':1821 '
Windows (Command Prompt)
netstat -ano | findstr :1821
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 1821 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 1821
From Windows
Test-NetConnection HOST -Port 1821
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 1821

Block port 1821 in the firewall

ufw
sudo ufw deny 1821/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 1821 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 1821 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 1821' -Direction Inbound -Protocol TCP -LocalPort 1821 -Action Block
ufw
sudo ufw deny 1821/udp
iptables
sudo iptables -A INPUT -p udp --dport 1821 -j DROP
nftables
sudo nft add rule inet filter input udp dport 1821 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 1821' -Direction Inbound -Protocol UDP -LocalPort 1821 -Action Block

Related ports

1819plato-lm1820mcagent1822es-elmd1823unisys-lm

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