Sockets

An endpoint for communication between two machines over a network.

Identified by: IP address + Port number = Socket Example: 192.168.1.1:8080

A connection = pair of sockets (client socket + server socket)

Types of Sockets

Type	Protocol	Characteristics
Stream Socket	TCP	Reliable, ordered, connection-oriented
Datagram Socket	UDP	Unreliable, connectionless, fast
Raw Socket	IP/ICMP	Direct access to lower layers, used in tools like ping

Socket Lifecycle — Server vs Client

SERVER                          CLIENT
  │                                │
socket()                        socket()
  │                                │
bind()                             │
  │                                │
listen()                           │
  │                                │
accept()  ◄────── connect() ───────┤
  │                                │
read()/write() ◄──────────────► read()/write()
  │                                │
close()                         close()

Key calls:

• socket() — create socket
• bind() — attach to IP + port (server only)
• listen() — wait for connections (server only)
• accept() — accept incoming connection (blocks until client connects)
• connect() — client initiates connection
• read()/write() or send()/recv() — exchange data
• close() — terminate

Port Numbers

Range	Type	Example
0–1023	Well-known	80 (HTTP), 443 (HTTPS), 22 (SSH)
1024–49151	Registered	3306 (MySQL), 6379 (Redis)
49152–65535	Ephemeral	Assigned to clients dynamically

When your browser connects to google.com:443 — Google uses port 443, your browser gets a random ephemeral port.

WebSockets (Different Thing)

• Built on top of HTTP, then upgrades to persistent connection.
• Full duplex — server can push data to client anytime.
• Used in: chat apps, live feeds, collaborative tools.
• Regular sockets are OS-level; WebSockets are application-level protocol.

Key Concepts

Blocking vs non-blocking:

• Blocking — accept() waits until a client connects, halts execution.
• Non-blocking — returns immediately, you poll or use callbacks.

Multiplexing:

• select() / epoll() — monitor multiple sockets at once.
• How Node.js handles thousands of connections on a single thread.