Computer Science

• Accessing a website in New York from a computer in London

  1. Fibre optic cable (made out of glass, size of a hair) under the Atlantic Ocean, and under the pavements, connects everywhere together

  2. Everywhere has an individual IP address, when you search a URL you are sending a request to that server's IP address, the server reads this, finds the website and then pulverises it into hundreds of tiny packets, and sends them off through any route possible

  3. The Internet works at the speed of light
• Network


  Two or more computers connected together with the ability to transmit data between each other
• Types of networks


  • Local area networks (LANs)
  • Wide area networks (WANs)
• LAN


  Network which is spread over a small geographical area or a single site
• WAN


  Network which is spread over a large geographical area
• Types of network topology

  • Physical topology
  • Logical topology
• Physical topology


  The physical layout of the wires and components which form the network
• Logical topology


  The layout which shows how data flows
• The largest public network in the world is called the Internet, and is similar in concept to a network you would find at home. This is known as a WAN (Wide Area Network). It's called the Internet as it is a network of Interconnected Networks
• World Wide Web


  A collection of resources accessed via the Internet
• Circuit Switching


  When you connect to somebody else on an old fashioned phone, you own the circuit of the wire between you. You are the only circuit on that line. Similar to train system
• Packet Switching

  Similar to road system -> you don't own the piece of road you're using, you share with other cars. All data shares its cable with all data on that line
• Internet backbone

  A set of dedicated connections that connect several large networks at various points on the globe
• Internet Service Providers (ISPs)

  Provide access to individual end-users. You don't connect to the Internet, you connect to the ISP which is connected to the Internet. The ISPs ration your Internet usage
• All ISPs have a connection (often to LINX, London Internet Exchange)
• IP (Internet Protocol) addressing system

  Used to uniquely identify each device on a network so that data can be sent to the correct destination
• IPv4


  Uses four numbers between 0 and 255, each number has a full stop between them. Every local router uses the same IP Address locally, however it has a unique number that is public facing. IP addresses in an area will all start with the same few numbers, as addressing is done regionally
• IPv6

  Uses 16 numbers rather than 4
• Uniform Resource Locator (URL)

  Used to specify the means of accessing a resource across a network and its location. The protocol and the domain name of the resource together form the URL
• Domain Name System (DNS)

  Dedicated computers with an index of domain names and their corresponding IP addresses. When a computer queries a DNS server for a domain name, the server returns an IP address that the computer can use to send a message to it
• DNS lookup process

  Attempts to look in the local browser cache

  2. If cache is empty, the DNS recursive resolver finds the DNS setting and keeps looking through different DNS Levels until it finds what it's looking for

  3. The root server resolves the request to see which top level server is needed

  4. Goes to the authoritative name server for that URL to get the IP address

  5. The IP address is sent back to the client
• DNS server structure

  There are 13 root DNS servers that work together to catalogue every domain name. These are segmented into geographical groupings or levels. When the IP address of a given domain is not known it is referred to a related domain server that may know. There are Generic TLDs and Country TLDs
• IP address

  A Unique Identifier for each computer using the Internet Protocol to communicate over a network. Its main functions are to identify a host or network interface and provide the location of the host within the network
• Static IP addressing

  Assigns a fixed, unchanging IP address to each device on a network
• Dynamic IP addressing


  Assigns IP addresses on a first-come, first-served basis from a pool of available addresses, using the Dynamic Host Configuration Protocol (DHCP)
• Static IP addressing is commonly used for network devices that offer services accessed by other network devices, while dynamic IP addressing is typically used for end-user devices
• Connecting to a website

  Device sends a request to a DNS server to find the IP address associated with the website's domain name

  2. DNS server responds with the corresponding IP address

  3. Device sends a data packet to the provided IP address, requesting the website's data

  4. The server at that IP address sends back the requested data, which the device assembles and displays as a website
• IPv4

  Internet Protocol version 4, uses a 32-bit address scheme resulting in around 4.3 billion possible unique IP addresses, expressed as four numbers separated by periods, with each number ranging from 0 to 255
• IPv6


  Developed to address the IPv4 address shortage, uses a 128-bit address scheme, addresses are divided into eight groups of four hexadecimal digits, separated by colons
• Internet registries


  Five global registries responsible for allocating IP addresses to specific domain names
• Domain names must be unique otherwise DNS requests could be confused
• Available domain names are issued by Internet registrars
• You can use www.1and1.co.uk/domaincheck to check prices of domain names
• In the command prompt you can input tracert and then the website name eg tracert google.com, and you will be able to see every journey the data makes, the IP addresses and the time it takes
• Standalone computer


  One computer, not connected to any other computing device
• Local Area Network (LAN)


  Two or more computers connected together within a small geographical area, for example confined to one building or site. The LAN is under your control, so if you have any problems, you can sort it out yourself
• Network topologies


  • Bus topology
  • Star topology
• Bus topology


  • An arrangement where nodes are connected in a daisy chain by a single central communications channel. Only one device can send a request at once, so more devices means more collisions. Cheaper and easier to connect new devices, but if one cable is disconnected all devices stop working, and there is a limit to the number of devices
• Star topology


  • An arrangement where a central node or hub provides a common connection point for all nodes. Each device can send a request whenever it wants, without being blocked. More complicated and expensive to set up, but less susceptible to issues as one cable break only affects one machine, and the speed is not affected by other devices. Switches are used instead of hubs as they are more active and logical
• Wireless network


  Requires a Wireless Access Point (WAP) which broadcasts on a fixed frequency, with which all devices within range can connect