A network server is a service that listens on a socket for inbound connections.
A Web/HTTP Server listens for TCP connection on the HTTP/HTTPS port (80) and expects to receive in the body of the packet a HTTP request in a text format.
A SMTP server listens for TCP connection on the submission port (25,487, 587) and expects to receive in the body of the packet a SMTP command, the EHLO command as first.
To create a simple server that will listen and write to the standard output, you can use NetClient such as:
With computer language, you can use a library that manages the wire for you. ie:
Example: https://netty.io/