How Data Is Transferred on the Internet

Web server

A web server is a system (software and hardware) that hosts websites and makes them accessible over the internet. It listens for incoming requests from web clients, processes them, and responds by sending the requested resources.

• A service that runs permanently on computer
• Hands domain name to OS and generates HTTP request and passes to TCP

Apache

An open source web server that listens on specific ports, processes requests, and sends responses.

• Widely used to process, delivery, and store website pages

Data packets

Data packets are small units of data transmitted over a network. They ensure efficient and reliable data transfer.

• They are useless on their own - travel over IP

Structure

• Header: contains metadata such as the source and destination IP addresses, packet number, and error checking data/checksum
• Payload: the actual data being sent, which is normally limited in size (e.g. part of a webpage, email, video stream)
• Trailer (footer): contains error detection information to ensure data integrity

Packet switching

Packet switching allows the network's resources to be shared among multiple users - dynamically allocates bandwidth as needed.

• Data is broken into packets
• Each packet is routed independently through multiple networks, and may go through different routes depending on traffic, availability and efficiency
• Packets may arrive out of order and are reassembled at the destination
• If a packet is lost, the system requests it again (TCP ensure retransmission)

Example: When a website is visited, the browser requests data. The server sends this data in multiple packets. If some packets arrive out of order, TCP will rearrange them correctly before rendering the page.

Real world applications

• Every piece of data sent over the internet uses packet switching
• 4G LTE and 5G cellular networks use packet switching, enabling services like video streaming, online gaming or video conferencing
• When an email is sent, its divided into packets, sent, then reassembled

TCP and UDP

Both transmission control protocol and user datagram protocol are communication protocols, operated at the transport layer of the OSI model.

• TCP
  • Connection-oriented
  • Reliable (ensures delivery)
  • Slower due to error checking
  • Establishes a reliable connection before transmitting data
  • Ensures correct sequence
  • Used for web browsing, emails, file transfers
• UDP
  • Connectionless
  • Unreliable (no guarantee of delivery)
  • Faster due to no error checking
  • Uses checksum, but doesn't request retransmission
  • No ordering guarantee
  • Used for streaming, gaming, video

Network nodes

A network node is any device that sends, receives or processes data in a computer network.

Types

• End nodes (host - where data originates from)
  • Example: computers, smartphones, servers, etc
• Intermediate nodes (networking devices, which help transmit data between end nodes)
  • Routers - direct data between networks using IP address
  • Switches - direct traffic within the same network (LAN)
  • Modems - convert digital signals (computer) into analog signals (telephone lines)

Internet protocol (IP) addresses, including IPv4

Internet Protocol is the principle communication protocol for relaying packets of data across network boundaries. IP addresses are unique numerical identifiers assigned to devices for network communication.

Example: A laptop connecting to a website uses its IP address to request data from the server's IP.

• IPv4
  • 32-bit address length
  • Dotted decimal format (192.168.1.1)
  • ~4.3 billion total addresses
  • Still widely used
• IPv6
  • 128-bit address length
  • Hexadecimal format (2001:db8::1)
  • Virtually unlimited total addresses
  • Increasing usage

NAT

• Many devices connecting to one modem/router, which then connects to the internet from one IP address

Public vs private IP addresses

• Public IP addresses are assigned by ISPs and are unique on the internet
• Private IP addresses are used within local networks, making them not accessible on the internet

Static vs dynamic IP addresses

• Static IP addresses are permanent, often used for web servers
• Dynamic IP addresses change periodically, and are assigned by DHCP (dynamic host configuration protocol)

Domain name systems (DNS)

DNS is the system that translates human-readable domain names into IP addresses.

• DNS servers resolve domain names to IP addresses
• Acts as an address book of the internet to map domain names to their respective servers

Summary: DNS is like a phone book - instead of remembering the phone number (IP address) for every website, we lookup the website's name, and DNS translates the name to IP address.

DNS record types

• A Record: Maps a domain to an IPv4 address
• AAAA Record: Maps a domain to an IPv6 address
• CNAME Record: Alias of another domain (blog.example.com → example.com)
• MX Record: Specifies mail servers for email routing

The combination of IP and DNS ensures that devices can communicate over the internet. DNS simplifies the user experience and IP addresses allow for efficient and direct data transmission across the network.

Summary - how data travels when you visit a website

• Enter a URL
• DNS resolves the domain to an IP address
• Computer sends HTTP requests to the server
• Data is broken into packets and transmitted using TCP/IP
• Packets travel through multiple routers to reach the destination
• Server processes the request and sends packets back
• Packets are reassembled to display the web page