Sockets in Action

An IP address plus a port, bound together inside a program, is a socket. A server listens on a socket; a client connects to it. That is layer 4 made concrete.

You can watch it happen with two tools you already have: python3 -m http.server PORT starts a listening server, and nc -z host port knocks on a port and reports (via its exit status) whether anything answered.

Servers take a moment to come up, so real scripts retry the knock a few times before concluding anything.