Ports & Sockets

// Well-known ports (0–1023):
// 20/21  — FTP (data/control)
// 22     — SSH
// 25     — SMTP
// 53     — DNS (UDP + TCP)
// 80     — HTTP
// 110    — POP3
// 143    — IMAP
// 443    — HTTPS (HTTP over TLS)
// 3306   — MySQL
// 5432   — PostgreSQL
// 6379   — Redis
// 8080   — HTTP alternate (dev servers, Spring Boot default)
// 9092   — Apache Kafka

// Java: bind server to a specific port
ServerSocket server = new ServerSocket(8080);   // privileged port: needs root if < 1024
System.out.println("Listening on port 8080");

Socket client = new Socket("api.example.com", 443);
int localPort = client.getLocalPort();          // OS assigned ephemeral port, e.g. 54231
int remotePort = client.getPort();              // 443
System.out.println("Ephemeral port: " + localPort);

// How one server port handles many clients:
//
// Connection 1: TCP, 10.0.0.1:443 ↔ 203.0.113.10:54231
// Connection 2: TCP, 10.0.0.1:443 ↔ 203.0.113.20:61002
// Connection 3: TCP, 10.0.0.1:443 ↔ 198.51.100.5:49812
//
// All three arrive at port 443, but the OS uses the full 5-tuple
// to dispatch each packet to the correct socket/thread.

// Java NIO — non-blocking server that handles many clients on one thread:
ServerSocketChannel serverChannel = ServerSocketChannel.open();
serverChannel.bind(new InetSocketAddress(8080));
serverChannel.configureBlocking(false);

Selector selector = Selector.open();
serverChannel.register(selector, SelectionKey.OP_ACCEPT);

while (true) {
    selector.select();                          // blocks until a channel is ready
    for (SelectionKey key : selector.selectedKeys()) {
        if (key.isAcceptable()) {
            SocketChannel client = serverChannel.accept();
            client.configureBlocking(false);
            client.register(selector, SelectionKey.OP_READ);
        } else if (key.isReadable()) {
            // handle read on this client socket
        }
    }
}