Network fundamentals

A2.1.1 Purpose and characteristics of networks

A computer network connects devices so that they can share resources and information, such as files, printers, apps and internet access.

Latency is the delay for data to travel across a network. Bandwidth is how much data it can carry each second.

A classic WAN has its own long-distance links. Most modern WANs use the internet instead, with VPNs keeping the traffic private, which is far cheaper.

A2.1.2 Modern digital infrastructures

BitTorrent and blockchains are distributed systems. Content delivery networks spread files around the world, and streaming services place servers in major cities as a form of edge computing.

Next: network devices →

A2.1.3 Network devices

A home “router” is really several logical devices in one box. Learn each device by the problem it solves.

Next: protocols →

A2.1.4 Network protocols

A protocol is a set of rules for how data is sent and received.

TCP and UDP

IP gets data from one computer to another. TCP and UDP get it from one application to another using port numbers, a bit like flat numbers in a building.

Choose a protocol.

import socket

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:   # IPv4 + TCP
    s.bind(("0.0.0.0", 65432))
    s.listen()
    conn, addr = s.accept()              # waits for a client
    with conn:
        print("Connected by", addr)
        while True:
            data = conn.recv(1024)       # up to 1024 bytes
            if not data:
                break
            conn.sendall(data.decode().upper().encode())

import socket

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.connect(("127.0.0.1", 65432))      # 127.0.0.1 is this computer
    s.sendall("hello, server".encode())
    print(s.recv(1024).decode())

HTTP and HTTPS

HTTP is the protocol of the web. It is stateless and human-readable. A client sends a request and the server replies with a status code, such as 200 (OK) or 404 (not found).

GET /index.html HTTP/1.1
Host: www.example.com

HTTP/1.1 200 OK
Content-Type: text/html

<h1>Welcome</h1>

import requests

response = requests.get("https://www.example.com")
print(response.status_code)
print(response.text[:200])

HTTPS adds encryption, normally TLS, authentication through a server certificate, and integrity, so that changes in transit are detected. It protects against eavesdroppers and man-in-the-middle attacks.

DHCP

DHCP gives a device an IP address when it joins a network. Addresses are leased for a set time, so they can be reused.

Next: the TCP/IP model →

Next: review →

HL A2.1.5 The TCP/IP model

The TCP/IP model splits networking into four layers, each with one responsibility. It has largely replaced the older OSI model.

Next: review →

Quick quiz

Exam-style questions

These are written for this site in the style of Paper 1. Answer on paper first, then open the mark points.

1. Describe one difference between a LAN and a WAN. [2]

   Mark points

   • A LAN covers a small area such as a building [1].
   • A WAN connects networks over a large area such as several cities [1].

2. Discuss one benefit and two limitations of cloud computing for a small business. [3]

   Mark points

   • Benefit: it can scale without buying hardware, which lowers cost [1].
   • Limitation: it depends on reliable, fast internet [1].
   • Limitation: data on third-party systems raises security and privacy concerns [1].

3. Explain why a video service caches popular films in local servers. [2]

   Mark points

   • Data has less distance to travel, so latency falls [1].
   • Less traffic reaches the main servers, which saves bandwidth [1].

4. Outline the role of a router, a switch and a modem in a home network. [3]

   Mark points

   • The switch connects devices within the LAN [1].
   • The router forwards packets between the LAN and the internet [1].
   • The modem converts between digital signals and the form used on the line [1].

5. Compare TCP and UDP. [4]

   Mark points

   • TCP is connection-oriented, while UDP is connectionless [1].
   • TCP guarantees ordered delivery with sequence numbers and acknowledgements [1].
   • UDP has less overhead, so it is faster [1].
   • UDP can lose or reorder data, which suits live streaming, while TCP suits file transfer [1].

6. Describe three features HTTPS adds to HTTP. [3]

   Mark points

   • Encryption stops eavesdroppers from reading the data [1].
   • Certificates authenticate the server [1].
   • Integrity checks detect changes in transit [1].

7. Outline how a laptop gets an IP address when it joins a network. [3]

   Mark points

   • It broadcasts a DHCP Discover message [1].
   • A server sends an Offer of an address from its pool [1].
   • The laptop Requests that address and the server Acknowledges it, giving a lease [1].