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Port 421 (ariel2)

IANA assigns port 421 to ariel2: Ariel 2.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
ariel2 TCP Ariel 2 - -
ariel2 UDP Ariel 2 - -

Check what is listening on port 421

Linux: is anything listening on TCP 421?
sudo ss -tlnp 'sport = :421'
Linux: UDP 421
sudo ss -ulnp 'sport = :421'
Linux / macOS: which process owns the port
sudo lsof -nP -i :421
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':421 '
Windows (Command Prompt)
netstat -ano | findstr :421
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 421 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 421
From Windows
Test-NetConnection HOST -Port 421
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 421

Block port 421 in the firewall

ufw
sudo ufw deny 421/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 421 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 421 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 421' -Direction Inbound -Protocol TCP -LocalPort 421 -Action Block
ufw
sudo ufw deny 421/udp
iptables
sudo iptables -A INPUT -p udp --dport 421 -j DROP
nftables
sudo nft add rule inet filter input udp dport 421 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 421' -Direction Inbound -Protocol UDP -LocalPort 421 -Action Block

Related ports

419ariel1420smpte422ariel3423opc-job-start

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