Port Ref — Network Port Reference

Is Port X Open? — Debugging Tips

Common commands to check if a specific port is open or in use on your system.

Check if a port is listening (Linux/macOS)

ss -tlnp | grep :PORT        # Linux — show TCP listeners
netstat -tlnp | grep :PORT   # Linux (legacy)
lsof -i :PORT                # macOS / Linux — who owns the port
sudo lsof -iTCP:PORT -sTCP:LISTEN  # macOS — TCP listeners only

Check if a remote port is open

nc -zv HOST PORT              # Netcat — quick TCP check
telnet HOST PORT              # Classic connectivity test
curl -v telnet://HOST:PORT    # Curl-based check
nmap -p PORT HOST             # Nmap — detailed port scan

Check from Windows

netstat -an | findstr :PORT   # Show connections on a port
Test-NetConnection HOST -Port PORT   # PowerShell
tnc HOST -Port PORT           # PowerShell (short alias)

Docker port mapping

docker port CONTAINER         # Show all mapped ports
docker ps --format "table {{.Names}}\t{{.Ports}}"  # List port mappings
ss -tlnp | grep docker-proxy  # See what Docker is using on host

Common issues

Port already in use: Another process is bound to the port. Use lsof -i :PORT to find it, then kill PID.

Connection refused: The port is not listening. Check if the service is running.

Connection timed out: A firewall is blocking the port. Check iptables -L or cloud security groups.

Ephemeral port conflicts: Ports 49152-65535 are used for outbound connections. Don't bind services there.