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Port 1991 (stun-p2)

IANA assigns port 1991 to stun-p2: cisco STUN Priority 2 port.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
stun-p2 TCP cisco STUN Priority 2 port - -
stun-p2 UDP cisco STUN Priority 2 port - -

Check what is listening on port 1991

Linux: is anything listening on TCP 1991?
sudo ss -tlnp 'sport = :1991'
Linux: UDP 1991
sudo ss -ulnp 'sport = :1991'
Linux / macOS: which process owns the port
sudo lsof -nP -i :1991
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':1991 '
Windows (Command Prompt)
netstat -ano | findstr :1991
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 1991 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 1991
From Windows
Test-NetConnection HOST -Port 1991
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 1991

Block port 1991 in the firewall

ufw
sudo ufw deny 1991/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 1991 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 1991 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 1991' -Direction Inbound -Protocol TCP -LocalPort 1991 -Action Block
ufw
sudo ufw deny 1991/udp
iptables
sudo iptables -A INPUT -p udp --dport 1991 -j DROP
nftables
sudo nft add rule inet filter input udp dport 1991 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 1991' -Direction Inbound -Protocol UDP -LocalPort 1991 -Action Block

Related ports

1989tr-rsrb-p3, mshnet1990stun-p11992stun-p3, ipsendmsg1993snmp-tcp-port

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