1. hello (serial)

   #1 of the "primitive networking servers series". A serial (non-concurrent)
   TCP server that just greets any connections with  "Hello, world!" before
   closing them.
module hello;

import std.socket : InternetAddress, Socket, TcpSocket, SocketOptionLevel, SocketOption;
import std.stdio : writefln;

enum port = 4444;

void main() {
    TcpSocket listener = new TcpSocket;
    listener.setOption(SocketOptionLevel.SOCKET, SocketOption.REUSEADDR, 1);
    listener.bind(new InternetAddress(port));
    writefln!"Listening on %d."(port);

    while (true) {
        Socket client = listener.accept;
        writefln!"Received connection from %s."(client.remoteAddress.toString);
        client.send("Hello, world!\n");

Server interaction

$ dmd -run hello.d
Listening on 4444.
Received connection from
Received connection from
Received connection from

Client interaction

$ # using nmap's ncat, rather than GNU's or OpenBSD's netcat
$ nc --recv-only localhost 4444
Hello, world!
$ nc --recv-only localhost 4444
Hello, world!
$ nc --recv-only localhost 4444
Hello, world!

sockets? binding? listening? what?

Beej's Guide to Network Programming is a great introduction to these concepts.

is that REUSEADDR stuff necessary?

The server will work without it, but you'll get an error like the following on server restarts:
std.socket.SocketOSException@std/socket.d(2825): Unable to bind socket: Address already in use
??:? @trusted void std.socket.Socket.bind(std.socket.Address) [0x55cbfd41dc28]
??:? _Dmain [0x55cbfd3f6a92]
This StackOverflow Answer explains what's going on there.

can I connect to this server remotely?

Firewalls permitting, yes: the port-only constructor of InternetAddress binds to ADDR_ANY. If you want to only bind to localhost for these tutorial servers, you can bind to new InternetAddress("", port) instead.

is this server fit for purpose?

For well-behaved peers... probably? It accepts a connection (blocks, can throw an exception), prints some information about it (can throw an exception), sends a string to it (may return an error code, may block), and then closes it (infallibly).