Web & Networking › Networking Fundamentals

Port

A number identifying a specific service on a host.

Also known as: port number, network port, TCP port, listening port

A port is a number from 0 to 65535 that identifies a specific service on a machine. The IP address gets traffic to the right computer; the port gets it to the right program on it.

192.168.1.20:5432      <- host : port  (here, a database)
https://example.com    <- port 443 is implied
http://localhost:3000  <- your dev server

Well-known ports

Defaults are conventions. Any service can be configured to use another port.

Everyday use

• A server listens on a port; a client connects to it. Only one program can listen on a given port and address at a time.
• That gives the classic error: “Address already in use” (EADDRINUSE in Node). Something is already running there, often a forgotten dev server. Find it:

lsof -i :3000          # macOS/Linux: which process uses port 3000?

then stop it, or choose another port.

• Firewalls and cloud security groups allow or block traffic per port. “Connection refused” often means nothing is listening there. A timeout often means a firewall is dropping the traffic.
• On Linux and macOS, ports below 1024 typically need elevated privileges to listen on. That’s why dev servers use ports like 3000 or 8080.
• Containers map ports: docker run -p 8080:80 exposes the container’s port 80 as 8080 on your machine.

See also IP addresses and inspecting connections.