The big idea: Every network device answers one question about the traffic reaching it.
"Which machine in here?" "Which network out there?" "Should this be allowed through at all?" Learn the question and the device follows.
Why the layer matters: A device can only act on what it can read.
A switch reads addresses inside one network; a router reads IP addresses across networks. That is why they are different boxes.
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Inside one network
- Switch — sends a frame to the one device it is for, inside a LAN
- Network interface card — connects one device to the network
- Wireless access point — lets devices join that LAN without cable
Between networks
- Router — reads the IP address and chooses the next network to send to
- Gateway — joins two networks that speak differently, translating between them
- Modem — converts between the network's digital signal and whatever the line carries
Deciding what is allowed
- Hardware firewall — inspects traffic and blocks anything the rules forbid
- Sits where the network meets the outside world
- Works on both incoming and outgoing traffic
Switch or router — the one distinction to be sure of: A switch moves traffic within one network. A router moves it between networks.
The box in a home does both, plus a modem and a wireless access point — which is why the distinction is easy to lose.
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The four layers, with the protocols and devices that work at each. Step through them.
Interactive diagram
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Reading the stack: Network access — switches, network interface cards, wireless access points, modems. They move bits and frames inside one network.
Internet — routers and gateways. They read IP addresses and choose routes.
A firewall can inspect several layers, which is exactly why it can enforce useful rules.
Why routers cannot be replaced by switches: A switch has no idea what an IP address is, so it cannot tell whether traffic is meant for another network.
Every message leaving your network needs something that reads IP — and that is a router.
How this is tested — you must say what each device DOES to the traffic, not just where it is plugged in. It comes up two ways:
Paper 1 Section A
- Describe the function of named devices, 2-4 marks
- Say which device a scenario needs
- Map devices to TCP/IP layers
Paper 1 Section B — case study
- Identify the devices in the case study's network
- Explain what each contributes
The classic trap: Using switch and router interchangeably. A switch works inside one network and cannot read IP addresses; a router works between networks and does nothing else.
A small office has twelve computers, shared internet access and staff using laptops without cables. Describe the network devices it needs and the function of each.
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