Port 11720 (h323callsigalt)

IANA assigns port 11720 to h323callsigalt: H.323 Call Control Signalling Alternate.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference

h323callsigalt TCP H.323 Call Control Signalling Alternate ITU-T -

h323callsigalt UDP H.323 Call Control Signalling Alternate ITU-T -

Registry dates for this port: last modified 2013-01-31.

Linux: is anything listening on TCP 11720?

sudo ss -tlnp 'sport = :11720'

Linux: UDP 11720

sudo ss -ulnp 'sport = :11720'

Linux / macOS: which process owns the port

sudo lsof -nP -i :11720

Older Linux / BSD (net-tools)

sudo netstat -tlnp | grep ':11720 '

Windows (Command Prompt)

netstat -ano | findstr :11720

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 11720 -State Listen | Select LocalAddress,OwningProcess

From another machine

nc -vz -w 5 HOST 11720

From Windows

Test-NetConnection HOST -Port 11720

UDP probe (no reply does not prove it is closed)

nc -vzu -w 5 HOST 11720

ufw

sudo ufw deny 11720/tcp

iptables

sudo iptables -A INPUT -p tcp --dport 11720 -j DROP

nftables

sudo nft add rule inet filter input tcp dport 11720 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block TCP 11720' -Direction Inbound -Protocol TCP -LocalPort 11720 -Action Block

ufw

sudo ufw deny 11720/udp

iptables

sudo iptables -A INPUT -p udp --dport 11720 -j DROP

nftables

sudo nft add rule inet filter input udp dport 11720 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block UDP 11720' -Direction Inbound -Protocol UDP -LocalPort 11720 -Action Block

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