Two addresses on one parcel
A parcel for a friend in another city carries a full street address. At each depot, the staff also need to know the next stop on the lorry route. Two kinds of address do two different jobs.
Data sent over a computer link works the same way. It has one address for the whole trip and another for the next stop.
Key words
| Word | What it means | Example |
|---|---|---|
| IP address | a number that names a device across networks | 10.0.12.1 |
| MAC address | a number built into a network card, used on one link | c6:a2:f6:cd:d4:eb |
| Packet | a piece of data with an IP address on it | |
| Frame | a piece of data with a MAC address on it, ready for one link | |
| Bit | one 1 or 0, the smallest piece of data | |
| Protocol | a set of rules computers follow to talk to each other | |
| Router | a device that joins networks by IP address | |
| Switch | a device that joins devices on one link by MAC address | |
| Ethernet | the usual wired way to join devices in a building | |
| Network | devices joined so they can share things | |
| OSPF | a protocol that lets routers share a map to find the best path |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can say what the network, data link and physical layers do and which devices work at each.
Starter
Have a go
Starter
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Layer 3 picks the route
The network layer (layer 3) gives each device an IP address and picks the route across many networks. Data here is a packet.
IP is the main protocol. ICMP carries error messages and the ping test. Routing protocols such as OSPF help routers learn routes. A router works at this layer.
Layer 2 moves data over one link
The data link layer (layer 2) moves data between two devices on the same link. Data here is a frame, with a MAC address for the next device. It also adds a check value to catch damage.
Ethernet and Wi-Fi work here. A switch works at this layer. A packet gets a new frame at every hop.
Extra: Find the MAC address, a layer 2 address
sudo ip netns exec h1 ip -br linkA command for the lesson’s lab terminal. It is shown here and not run.
Layer 1 sends the bits
The physical layer (layer 1) turns bits into signals: electricity in a copper cable, light in fibre, radio in Wi-Fi. It also covers the plugs, the cables and the voltages.
If a cable is cut, the fault is at layer 1. Nothing above it can work.
The bottom three layers
| Layer | Name | Data is called | Address used | Device | Examples |
|---|---|---|---|---|---|
| 3 | Network | Packet | IP address | Router | IP, ICMP, OSPF |
| 2 | Data link | Frame | MAC address | Switch | Ethernet, Wi-Fi |
| 1 | Physical | Bits | None | Cable, access point radio | Copper, fibre, radio |
The cells students fill in are left blank in this preview.