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Port 11 (systat)

IANA assigns port 11 to systat: Active Users.

Well-known (system) port, 0-1023 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
systat TCP Active Users Jon Postel -
systat UDP Active Users Jon Postel -

Check what is listening on port 11

Linux: is anything listening on TCP 11?
sudo ss -tlnp 'sport = :11'
Linux: UDP 11
sudo ss -ulnp 'sport = :11'
Linux / macOS: which process owns the port
sudo lsof -nP -i :11
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':11 '
Windows (Command Prompt)
netstat -ano | findstr :11
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 11 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 11
From Windows
Test-NetConnection HOST -Port 11
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 11

Block port 11 in the firewall

ufw
sudo ufw deny 11/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 11 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 11 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 11' -Direction Inbound -Protocol TCP -LocalPort 11 -Action Block
ufw
sudo ufw deny 11/udp
iptables
sudo iptables -A INPUT -p udp --dport 11 -j DROP
nftables
sudo nft add rule inet filter input udp dport 11 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 11' -Direction Inbound -Protocol UDP -LocalPort 11 -Action Block

Related ports

7echo9discard13daytime15Unassigned [was netstat]

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