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Port 1721 (caicci)

IANA assigns port 1721 to caicci.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
caicci TCP caicci Sylvia Scheuren -
caicci UDP caicci Sylvia Scheuren -

Check what is listening on port 1721

Linux: is anything listening on TCP 1721?
sudo ss -tlnp 'sport = :1721'
Linux: UDP 1721
sudo ss -ulnp 'sport = :1721'
Linux / macOS: which process owns the port
sudo lsof -nP -i :1721
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':1721 '
Windows (Command Prompt)
netstat -ano | findstr :1721
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 1721 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 1721
From Windows
Test-NetConnection HOST -Port 1721
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 1721

Block port 1721 in the firewall

ufw
sudo ufw deny 1721/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 1721 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 1721 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 1721' -Direction Inbound -Protocol TCP -LocalPort 1721 -Action Block
ufw
sudo ufw deny 1721/udp
iptables
sudo iptables -A INPUT -p udp --dport 1721 -j DROP
nftables
sudo nft add rule inet filter input udp dport 1721 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 1721' -Direction Inbound -Protocol UDP -LocalPort 1721 -Action Block

Related ports

1719h323gatestat1720h323hostcall1722hks-lm1723PPTP

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