Year 7 · Autumn 2
Networks · Lesson 4 of 5
IP addresses and packets: the packet-passing game
UnpluggedTopic 15 Networks and the internet
Year 7 · Networks · Lesson 4 of 5
You will learn to
- Explain why a file is split into numbered packets before it travels.
- Describe what a packet holds: who it is from, who it is to, a sequence number and some data.
- Explain what happens when a packet is lost or arrives out of order.
- Identify an IPv4 address and explain why each part stops at 255.
Year 7 · Networks · Lesson 4 of 5
Success criteria
You have got it when you can say:
- I can put packets back in order using their numbers.
- I can say what happened when our class lost a packet.
- I can explain why an IP address is like a postal address.
Year 7 · Networks · Lesson 4 of 5
Retrieval starter
Questions from before. Write your answers, then show them.
- Last lesson
In a star network the central switch breaks. What happens?
A Every device loses the networkB Only one device drops offC Data finds another route
A. Every device loses the network
- Last unit
Most of a smartphone's lifetime carbon footprint comes from…
A Charging itB Recycling itC Making it
C. Making it
- Earlier this unit
A game feels laggy, but big downloads are fast. The problem is most likely…
A Too many IP addressesB Low bandwidthC High latency
C. High latency
Year 7 · Networks · Lesson 4 of 5
Key words (1 of 3)
- IP address
- A device's address on a network. IPv4 = 4 bytes.
- packets
- A small numbered chunk of data sent across a network.
- LAN / WAN
- Local Area Network: a network over a small area.
- Router
- Connects different networks and forwards packets.
Year 7 · Networks · Lesson 4 of 5
Key words (2 of 3)
- MAC address
- Fixed unique ID of a network card. 48 bits in hex.
- DNS
- Domain Name System: turns website names into IP addresses.
- Packet
- A small numbered chunk of data sent across a network.
- Protocol
- A set of agreed rules for communication.
Year 7 · Networks · Lesson 4 of 5
Key words (3 of 3)
- Bandwidth / latency
- How much data a connection can carry per second.
The explanation · step 1 of 2
Addresses
IP address: a device's address on a network. It can change, like a postal address when you move.
- IPv4 looks like 192.168.1.25.
- Each part stops at 255 because each part is one byte.
- 4 bytes = 32 bits ≈ 4.3 billion addresses. Not enough, so IPv6 uses 128 bits.
MAC address: the fixed ID of the network card itself. 48 bits, written in hex: 3C:22:FB:7A:1E:90.
The explanation · step 2 of 2 · part 1 of 2
Packets
A photo is split into thousands of packets. Each has a header (from, to, sequence number), a slice of the data, and a check value.
- Split the file into numbered packets.
- Send each one separately.
- Routers send each packet on whatever route is least busy.
- Packets arrive at different times, out of order.
- The receiver puts them back in order by number.
- Anything missing gets asked for again.
The explanation · step 2 of 2 · part 2 of 2
Packets (continued)
Why chop it up? Many people share the cables at once. A broken link just means another route. A lost packet is a small resend, not the whole file.
Year 7 · Networks · Lesson 4 of 5
Try it: Send a message in packets
Year 7 · Networks · Lesson 4 of 5
Try it: Check an IP address
Year 7 · Networks · Lesson 4 of 5
Quick quiz · 1 of 3
A 400 MB file on a 100 Mbps line takes at best…
- A 4 seconds
- B 32 seconds
- C 40 seconds
B. 32 seconds
400 × 8 = 3,200 megabits. 3,200 ÷ 100 = 32 seconds.
Year 7 · Networks · Lesson 4 of 5
Quick quiz · 2 of 3
Why can't an IPv4 part be 300?
- A 300 is reserved
- B Routers can't count that high
- C Each part is one byte: max 255
C. Each part is one byte: max 255
8 bits hold 0 to 255.
Year 7 · Networks · Lesson 4 of 5
Quick quiz · 3 of 3
Which turns www.bbc.co.uk into an IP address?
- A SMTP
- B HTTP
- C DNS
C. DNS
DNS is the phone book.
Year 7 · Networks · Lesson 4 of 5
Exit ticket
Answer on paper before you go. Then check.
Packets arrive out of order. How are they fixed?
A The router sorts themB They're sent againC Sequence numbers
C. Sequence numbers The receiver puts them back in order by number.
Which device joins your home LAN to your ISP's network?
A RouterB Wireless access pointC Switch
A. Router A router joins different networks. A switch only joins devices inside one LAN.