Structure of the internet

• The internet is a global network of interconnencted networks that communicate using a common set of standards and protocols. These networks are owned and managed by a wide range of organisations, such as national governments, private companies, and academic institutions.
• Individuals and organisations connect to the internet through an internet service provider (ISP).
• In the home, your ISP supplies a device (e.g. a home hub) that provides an outgoing connection from your home network to that of the service provider. This combined device offers a wide range of network features. It usually has an Ethernet switch, with a few ports for wired devices, and a wireless access point for wireless connections. A router manages the outgoing connection and a modem (sometimes provided as a separate box) converts the signal to a suitable type for the outgoing media.
• The link from your home router to the internet is itself part of a larger network belonging to the ISP. It may cover a small geographic area, such as a town, but could extend many tens of miles to rural outposts. It may support a few hundred or several thousand users.
• In general, if routers are used to connect local area networks (LAN) together, a wide area network (WAN) is created.
• With the growth of the internet and its high speed communication links, many companies now connect to each of their offices over the internet using a virtual private network (VPN), to provide a secure connection between sites. A VPN allows users to send and receive data across public telecommunication networks as if their devices were directly connected to the private network, which drastically reduces the cost.
• If WANs are connected together, an increasingly complex topology of interconnected networks can be built. Routers play a crucial role in developing interconnected networks, as they are needed to join the networks together. These connections are a collection of Ethernet, ADSL, coaxial, fibre, 3G/4G, wireless, satellite, undersea cable, powerline transmission, and many other technologies.
• A router is a networking device that forwards data packets between computer networks. However, not all routers are the same.
• Edge routers link one network to another.

  • Subscriber routers is the term given to routers provided to homes and small businesses. These routers are small scale, low-cost devices with two interfaces (one for each network). Simple, low-cost routers can also be used on internal networks to link network segments (where the network has been subdivided into segments for security or performance reasons).
  • Enterprise routers connect large businesses and ISP networks to the internet. These routers are far more powerful (and expensive) and are capable of handling very high volumes of data.
• Core routers are the routers that are part of the internet backbone. These routers have multiple interfaces, all of which can work at the highest speed simulatenously.
• Traffic on the internet is transported as packets. An internet packet is made up of the data that is being transported, which is called the payload, and a header.
• Internet packets carry all sorts of payloads. For example, the data might be part of a web page, or an email, or a streamed audio track. The type of data is identified by the protocol field within the header. Internet packets have a maximum size to prevent anything 'hogging' the bandwidth.
• Packet switching


  A method used to move packets around the internet
• Packet switching

  • Uses relatively small data packets
  • Allows a data path to be shared
  • Different packets from the same 'conversation' may be sent over different routes
• End-to-end principle


  The endpoints (source and destination) are responsible for checking that everything that has been sent is received, as appropriate
• Most traffic over the internet uses packet switching
• In any packet switched network, the router is the key device. The router examines the destination address of an incoming packet and decides which of its interfaces the packet should be sent to, until it eventually arrives at the destination.
• Each router has a set of rules — a routing table — that it uses to decide what to do with an incoming packet. Each packet is treated individually and a separate routing decision is made for each one. Each router-to-router link is called a hop; the router determines the best 'next hop' to allow the packet to move on its way to its final destination. If any part of the communications infrastructure fails, the router chooses a different route and subsequent packets may be sent over a different path.
• Routers can be configured so that they select the best route or can share data with each other so that they learn the best routes. This is how the core routers work. The routing table is constantly updated with information about the optimal route for packets to take.
• At times, some routers may receive packets faster than they are able to route them on. These packets are buffered in memory and this introduces delays (referred to as a 'high latency'). For most traffic this is not an issue; it just means that the web page takes longer to load or a file download taking more time than expected. However, for other network traffic these delays may have more of an impact, particularly if you are talking over a voice call or playing an online game. If the buffering is severe, the router many run out of memory and packets are simply discarded.
• IP packets contain a Type of Service (ToS) field in the packet header; this makes it possible to mark packets with a priority level and thus request special treatment i.e. to be placed at the front of the queue of packets to be routed. However, routers may choose to implement or ignore these requests.
• A situation can occur where packets are sent to a destination address which is unreachable. Routers, being unaware of this, may route such packets towards a default device. This default device may also pass the packet on, causing a loop to occur. In such circumstances, a packet could loop forever between routers. To prevent this, internet packets have a time to live (TTL) counter in their header. This is initially set when the packet is created, and reduced by one every time it goes through a router. If the counter reaches zero, the packet is discarded.