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Port 20810 (crtech-nlm)

IANA assigns port 20810 to crtech-nlm: CRTech NLM.

Registered (user) port, 1024-49151 TCP

IANA assignment

Service nameProtoDescriptionAssigneeReference
crtech-nlm TCP CRTech NLM Cullimore And Ring Technologies -

Registry dates for this port: registered 2021-06-24.

Check what is listening on port 20810

Linux: is anything listening on TCP 20810?
sudo ss -tlnp 'sport = :20810'
Linux / macOS: which process owns the port
sudo lsof -nP -i :20810
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':20810 '
Windows (Command Prompt)
netstat -ano | findstr :20810
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 20810 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 20810
From Windows
Test-NetConnection HOST -Port 20810

Block port 20810 in the firewall

ufw
sudo ufw deny 20810/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 20810 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 20810 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 20810' -Direction Inbound -Protocol TCP -LocalPort 20810 -Action Block

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