Understand how the internet is structured (IP addressing, routers)

The idea

The internet is a network of networks: many independently owned networks are joined so that any device can exchange data with any other. Each device needs an address so data reaches the right destination. An IP address identifies a device or interface on a network; a public address is globally unique, while a private address is reused inside a local network and translated at the boundary. Data is split into packets, each carrying a destination IP address. Routers are the devices that read that destination address and forward each packet along the best available path, hop by hop, until it arrives. Because routers choose routes dynamically, traffic can take different paths at different times, and a packet may pass through several routers. A router also connects separate networks, for example a home network to an internet service provider.

What earns the marks

• An IP address uniquely identifies a device or interface so packets can be delivered to the correct destination.
• A router forwards packets between networks by reading the destination IP address and choosing a route.
• The internet is a network of networks, so data may cross several routers before reaching its destination.
• Packets may travel by different routes, so routers make forwarding decisions rather than using one fixed path.
• Private IP addresses are used inside local networks and are not directly reachable from the public internet.

Common mistakes and how to put them right

• Thinking a router stores the whole file.

  Put it right: Routers forward individual packets, and the destination device reassembles them.

• Confusing a router with a switch.

  Put it right: A switch forwards frames within one local network, while a router forwards packets between networks.

• Believing an IP address identifies a person.

  Put it right: It identifies a device or interface, and addresses can be shared or reassigned.

• Assuming all packets take the same path.

  Put it right: Routers can forward packets along different routes.

Exam tips

• Read the question stem for the key term, such as router or IP address, before looking at the options.
• Eliminate options that describe a different device, such as a switch or hub, rather than a router.
• Check whether the question asks about addressing, forwarding or structure, and choose the option that matches that focus.