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Port 21000 (irtrans)

IANA assigns port 21000 to irtrans: IRTrans Control.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
irtrans TCP IRTrans Control Marcus Mueller -
irtrans UDP IRTrans Control Marcus Mueller -

Registry dates for this port: registered 2004-11.

Check what is listening on port 21000

Linux: is anything listening on TCP 21000?
sudo ss -tlnp 'sport = :21000'
Linux: UDP 21000
sudo ss -ulnp 'sport = :21000'
Linux / macOS: which process owns the port
sudo lsof -nP -i :21000
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':21000 '
Windows (Command Prompt)
netstat -ano | findstr :21000
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 21000 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 21000
From Windows
Test-NetConnection HOST -Port 21000
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 21000

Block port 21000 in the firewall

ufw
sudo ufw deny 21000/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 21000 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 21000 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 21000' -Direction Inbound -Protocol TCP -LocalPort 21000 -Action Block
ufw
sudo ufw deny 21000/udp
iptables
sudo iptables -A INPUT -p udp --dport 21000 -j DROP
nftables
sudo nft add rule inet filter input udp dport 21000 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 21000' -Direction Inbound -Protocol UDP -LocalPort 21000 -Action Block

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