If you're learning backend development, you've probably seen things like: localhost:3000 localhost:5000 :80 :443 At first, the port number can seem like just another configuration value. But why does a server need a port when it already has an IP address? If one server is running a Node.js API, PostgreSQL, Redis, and NGINX at the same time, how does the operating system know which application should receive an incoming request? That's the problem ports solve. ...