Topologies and the internet
Star, mesh and bus
- Star: every device has its own cable to a central switch. One broken cable affects one device; a failed switch stops everything. n devices need n cables.
- Mesh: devices are linked to several others, so data can take another route if a link fails. A full mesh links every device to every other: n devices need n(n - 1) ÷ 2 links, which gets expensive fast. The internet is a partial mesh of routers.
- Bus: every device has a short drop cable to one shared main cable, the backbone, with a terminator at each end to absorb the signal. It is cheap (little cable, no switch), but all data shares one cable, so collisions grow as traffic grows, and a break in the backbone stops the whole network.
The internet, DNS and the cloud
The internet is a worldwide network of networks.
The World Wide Web is the collection of pages reached over it.
- You type a URL. The browser asks a DNS server for the IP address of the domain name.
- The DNS server looks it up (asking other DNS servers if it must) and sends the IP address back.
- The browser sends a request to the web server at that IP address.
- The server sends the page back.
Web hosting stores a website on a server connected to the internet. The cloud is remote servers that store data and run services: reachable anywhere, but you need a connection and someone else holds your data.
Addresses
- IPv4: 32 bits, written as four denary numbers 0 to 255, such as 192.168.4.17. Used to route packets between networks; can change when a device moves.
- IPv6: 128 bits, written as eight groups of hex digits, because IPv4 ran out of addresses.
- MAC address: 48 bits, six pairs of hex digits such as 3C:22:FB:9A:01:7E, fixed into the NIC when it is made; used on the local network.
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