Linux Networking & TCP/IP for Developers · Lesson

Introduction to Socket API

Explore the core concepts of socket programming, including socket types, addresses, and common API calls.

Introduction to Socket API is a free Linux Networking & TCP/IP for Developers lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Linux Networking & TCP/IP for Developers learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

What are Network Sockets?

Welcome to socket programming! A socket is a crucial concept in network communication. Think of it as one endpoint of a two-way communication link between two programs running on the network.

It's like a phone jack on a wall. You plug a phone into it to connect to the phone network. Similarly, programs use sockets to connect and talk over a computer network.

Python's Socket Module

Python makes socket programming accessible through its built-in socket module. This module provides all the necessary functions and classes to create and manage sockets.

You'll typically import it at the beginning of your script. Let's see how to get started:

import socket

# Now you can use socket functions!

Socket Address Families

Before creating a socket, we need to specify its address family. This determines the type of addresses the socket can work with.

socket.AF_INET: Used for IPv4 addresses (e.g., 192.168.1.1). This is the most common.
socket.AF_INET6: Used for IPv6 addresses.
socket.AF_UNIX: Used for communication between processes on the same machine.

We'll primarily use AF_INET for standard internet communication.

Socket Types: Stream vs. Datagram

Next, we choose the socket type, which defines the communication style.

socket.SOCK_STREAM: This type uses TCP (Transmission Control Protocol). It's reliable, ordered, and connection-oriented, like a phone call. Data arrives exactly as sent.
socket.SOCK_DGRAM: This type uses UDP (User Datagram Protocol). It's connectionless and less reliable, like sending postcards. Data might arrive out of order or not at all.

Most common applications, especially web browsing, use SOCK_STREAM.

Creating Your First Socket

Now let's put it together and create a basic socket object. We'll specify the address family (IPv4) and the socket type (TCP stream).

This line creates an inactive socket that's ready to be configured for either client or server roles.

import socket

# Create a TCP/IP socket
my_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

print(f"Socket created: {my_socket}")
# Don't forget to close sockets when done!
my_socket.close()

Binding a Socket (Server Side)

For a server, the socket needs an address to listen on. This is called binding. You assign an IP address and a port number to your socket.

Clients will use this address and port to connect to your server. A port is a number that identifies a specific application or service on a host.

import socket

HOST = '127.0.0.1'  # Localhost
PORT = 65432        # Port to listen on (> 1023 for non-privileged)

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    print(f"Socket bound to {HOST}:{PORT}")
    # The socket is now bound, but not yet listening or accepting.

Listening for Connections

After binding, a server socket must listen for incoming client connections. The listen() method puts the socket into server mode.

The argument to listen() is the backlog, which is the maximum number of unaccepted connections that the system will allow before refusing new connections.

import socket

HOST = '127.0.0.1'
PORT = 65432

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    s.listen(1) # Allow 1 pending connection
    print(f"Server listening on {HOST}:{PORT}")
    # Now the server is ready to accept a connection.

Accepting a Connection

When a client tries to connect, the server uses the accept() method. This method blocks (waits) until a client connects.

It then returns a new socket object (conn) representing the connection to the client, and the client's address (addr). All communication with the client happens via this new socket.

import socket

HOST = '127.0.0.1'
PORT = 65432

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    s.listen(1)
    print("Waiting for a client connection...")
    # This part would typically be in a loop in a real server
    conn, addr = s.accept()
    with conn:
        print(f"Connected by {addr}")
        # Now 'conn' can be used to send/receive data to/from the client.

Connecting to a Server (Client Side)

A client socket doesn't bind; instead, it uses the connect() method to establish a connection with a server that is listening on a specific IP address and port.

Once connected, the client can immediately start sending and receiving data. Data is always exchanged as bytes.

import socket

HOST = '127.0.0.1'
PORT = 65432

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    try:
        s.connect((HOST, PORT))
        print(f"Connected to server at {HOST}:{PORT}")
        s.sendall(b'Hello from client!')
        data = s.recv(1024) # Receive up to 1024 bytes
        print(f"Received: {data.decode()}")
    except ConnectionRefusedError:
        print("Error: Server not running or not listening.")

Sending and Receiving Data

Once a connection is established, data can be exchanged using sendall() and recv().

socket.sendall(data): Sends all of the byte-like data to the connected socket. It keeps sending until all data is sent or an error occurs.
socket.recv(bufsize): Receives data from the socket. bufsize is the maximum number of bytes to be received at once. It returns an empty bytes object (b'') when the connection is closed.

Socket API Quick Check

Which of the following statements about Python's socket module and socket types is TRUE?

Recap: Intro to Sockets

Great job! In this lesson, you've taken your first steps into socket programming with Python. You learned:

What a network socket is and its role in communication.
About Python's socket module.
The difference between address families (AF_INET, AF_INET6) and socket types (SOCK_STREAM for TCP, SOCK_DGRAM for UDP).
The basic API calls: socket() for creation, bind() and listen() for servers, and connect() for clients.
How accept() handles incoming connections and how sendall()/recv() work.

Next, we'll dive deeper into building full TCP client-server applications!

Frequently asked questions

Is the “Introduction to Socket API” lesson free?

Yes — the full text of “Introduction to Socket API” is free to read here on the web, and the Linux Networking & TCP/IP for Developers course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Linux Networking & TCP/IP for Developers course, upgrade to CoddyKit PRO.

What will I learn in “Introduction to Socket API”?

Explore the core concepts of socket programming, including socket types, addresses, and common API calls. You practise Linux Networking & TCP/IP for Developers with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Linux Networking & TCP/IP for Developers?

No prior experience is required. Linux Networking & TCP/IP for Developers on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Introduction to Socket API” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Linux Networking & TCP/IP for Developers lesson?

Yes. Every Linux Networking & TCP/IP for Developers lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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