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Port 21847 (netspeak-cs)

IANA assigns port 21847 to netspeak-cs: NetSpeak Corp. Connection Services.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
netspeak-cs TCP NetSpeak Corp. Connection Services Toby Hosterman -
netspeak-cs UDP NetSpeak Corp. Connection Services Toby Hosterman -

Check what is listening on port 21847

Linux: is anything listening on TCP 21847?
sudo ss -tlnp 'sport = :21847'
Linux: UDP 21847
sudo ss -ulnp 'sport = :21847'
Linux / macOS: which process owns the port
sudo lsof -nP -i :21847
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':21847 '
Windows (Command Prompt)
netstat -ano | findstr :21847
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 21847 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 21847
From Windows
Test-NetConnection HOST -Port 21847
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 21847

Block port 21847 in the firewall

ufw
sudo ufw deny 21847/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 21847 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 21847 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 21847' -Direction Inbound -Protocol TCP -LocalPort 21847 -Action Block
ufw
sudo ufw deny 21847/udp
iptables
sudo iptables -A INPUT -p udp --dport 21847 -j DROP
nftables
sudo nft add rule inet filter input udp dport 21847 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 21847' -Direction Inbound -Protocol UDP -LocalPort 21847 -Action Block

Related ports

21845webphone21846netspeak-is21848netspeak-acd21849netspeak-cps

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