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CSP LabBy Educator Today

Pilot · L10 · 60 minutes

Internet paths, packets and failure

Model a network as a graph, find valid paths, explain how shared open protocols support communication and growth, reassemble packets using sequence metadata, and test how redundancy keeps devices connected when links fail — while separating this simplified model from real protocol guarantees.

Topics: 4.1 The Internet · 4.2 Fault Tolerance

Goals and vocabulary

By the end you can:

  • Distinguish a computing device, a network, the Internet and the Web.
  • Identify valid paths in a network graph.
  • Explain how packets that take different paths are reassembled using sequence numbers.
  • Determine whether two devices stay connected after stated links fail, and name the surviving path.
  • Explain why redundancy improves fault tolerance without guaranteeing zero outages.
Computer network
Connected computing devices that can send and receive data.
Protocol
An agreed set of rules for how data is formatted and sent.
Path
A sequence of directly linked devices from a sender to a receiver.
Packet
A small piece of a message, with metadata such as its destination and sequence number.
Redundancy
Extra paths or parts, so the system still works when one fails.
Fault tolerance
The ability of a system to keep working when some parts fail.
Single point of failure
One part whose failure alone stops communication.

Devices, networks and the Internet

A computing device is a machine that runs programs, such as a phone, laptop or router. A computer network is a group of devices connected so they can send and receive data. The Internet is a network of networks.

Communication works only because devices follow shared protocols: agreed rules for formatting, addressing and sending data. The Internet’s protocols are open, meaning anyone may build a device or network that follows them. That is how it grew without one owner approving each new connection.

Easily confused terms
TermWhat it is
IP addressA numeric address used to deliver data to a device on the Internet.
Domain nameA human-readable name, such as school.example, that is looked up to find an address.
The InternetThe worldwide network of networks and its protocols.
The World Wide WebA system of linked pages and files that uses the Internet. It is one service on the Internet, alongside email, streaming and more.

A network as a graph

Worked example

We model a network as a graph: letters are devices, and each link connects two devices directly. This is a simplified teaching model, not a map of a real network.

Links in the model network
DeviceDirectly linked to
S (sender)A, B
AS, C, D
BS, C
CA, B, T
DA, T
T (receiver)C, D

A path is a sequence of devices where each neighbouring pair is linked. S → A → C → T is a path. S → C → T is not, because S and C are not directly linked. From S to T there are four paths that never revisit a device: the three short ones S-A-C-T, S-A-D-T and S-B-C-T, and the longer S-B-C-A-D-T.

Packets, different routes and reassembly

Worked example

Data is split into packets. Each packet carries metadata, including its destination and a sequence number. In our model, different packets of one message may take different paths and so arrive out of order.

A three-packet message in the model
PacketPath takenArrival order
1 of 3S-A-D-T2nd
2 of 3S-B-C-T3rd
3 of 3S-A-C-T1st

Packets arrive 3, 1, 2. The receiver uses the sequence numbers to put them back as 1, 2, 3. If packet 2 never arrives, the message is incomplete: the receiver knows from “of 3” that something is missing and cannot rebuild the whole message until it is delivered.

Redundancy and failure

Worked example

Redundancy means having more than one path, so the network keeps working when part of it fails. This is fault tolerance.

Failures in the model network
Failed link(s)Can S still reach T?Surviving path
C-TYesS-A-D-T
A-CYesS-A-D-T or S-B-C-T
S-A and S-BNoNone. S has no working links, so it is isolated.

When S-A and S-B both fail, adding a link elsewhere, such as B-D, does not help: S itself has no working connection. A single point of failure is one link or device whose failure alone disconnects part of the network.

Explore each concept

Choose a concept, change its inputs and follow the explanation. Each example models the stated rule or scenario; completing it is practice rather than evidence of mastery.

  • Devices, systems and networks
  • Open protocols and scalability
  • DNS naming versus trust
  • Web versus Internet
  • Reliable delivery versus best effort
Open this example on its own

Change it · step through · explain

Change an input, predict what will happen, then advance one step. Read why the state changes, reset and experiment. This fixed teaching model does not save progress or award XP.

Ready · Make a prediction

Ready to explore: DNS, IP, TCP/UDP and Web services
  1. ServicewebWaiting
  2. A packet is lostyesWaiting

Ready to explore: DNS, IP, TCP/UDP and Web services

Why: Change an input, predict the first effect, then advance one step.

Try this: Change one input or choose another listed case. Predict the effect, reset and compare the states and governing rule.

Step through and experiment

Change it · step through · explain

Change an input, predict what will happen, then advance one step. Read why the state changes, reset and experiment. This fixed teaching model does not save progress or award XP.

Ready · Make a prediction

Ready to experiment
  1. Input 14,1,3Waiting

Ready to experiment

Why: Predict the first change, then advance one step.

Try this: Compare 1,1,1,1 with 4,2,1,3. Both have four deliveries; only one has every position.

Step through and experiment

Change it · step through · explain

Change an input, predict what will happen, then advance one step. Read why the state changes, reset and experiment. This fixed teaching model does not save progress or award XP.

Ready · Make a prediction

Ready to experiment
  1. Input 1C-TWaiting
  2. Input 2(empty)Waiting

Ready to experiment

Why: Predict the first change, then advance one step.

Try this: Fail C-T, then also D-T. Add a link and predict whether it reconnects S to T.

Common misconceptions

  • The Web is the Internet

    The belief: The Internet and the World Wide Web are the same thing.

    What is actually true: The Web is one service that uses the Internet; email and streaming use it too.

    Check yourself: Does sending an email need a web page?

  • All packets travel together

    The belief: Every packet of a message follows the same route and arrives in order.

    What is actually true: In the model, packets can take different paths and arrive out of order; sequence numbers restore the order.

    Check yourself: What does the receiver use to put 3, 1, 2 back in order?

  • Redundancy means it never fails

    The belief: If a network has more than one path, it can never go down.

    What is actually true: Redundancy lowers the risk, but enough simultaneous failures can still disconnect a device.

    Check yourself: What happens if S-A and S-B both fail?

Guided practice

Guided: paths and failures

Use the model network table: S–A, S–B, A–C, B–C, C–T, A–D, D–T.

  1. List every path from S to T that does not visit a device twice.
  2. Packets arrive in the order 2, 3, 1. Write the reassembled order and explain how the receiver knows it.
  3. Link C-T fails. Which of your paths still work?
  4. Links S-A and S-B both fail. Can any added link between other devices reconnect S? Explain.

Independent practice

Independent: a second network

A new fictional network has links P–Q, Q–R, R–Z and P–M, M–Z.

Links
P–QQ–RR–ZP–MM–Z
  1. List the paths from P to Z.
  2. If Q–R fails, can P still reach Z? Give the surviving path.
  3. Add one link to this network and name a failure it would protect against.
  4. Explain one reason why even your improved network could still lose its P to Z connection.

Exit check

  1. Explain the difference between the Internet and the World Wide Web in one sentence.
  2. Give one failure in the model network that leaves S able to reach T, and one that does not.

Check yourself

Practice only: answers are checked in your browser and nothing is saved or sent to your teacher. Each explanation links back to the section it practises.

Lesson practice

  1. Question 1 of 6Which of these is a computer network rather than a single computing device?

    Select one answer.

  2. Question 2 of 6In the model network (S–A, S–B, A–C, B–C, C–T, A–D, D–T), which sequence is a valid path from S to T?

    Select one answer.

  3. Question 3 of 6Why can devices made by different companies communicate over the Internet?

    Select one answer.

  4. Question 4 of 6What is the relationship between a domain name such as school.example and an IP address?

    Select one answer.

  5. Question 5 of 6In the model, packet 1 travels S-A-D-T and packet 2 travels S-B-C-T. Which statement is correct?

    Select one answer.

  6. Question 6 of 6In the model, a message is split into packets labelled 1 of 3, 2 of 3 and 3 of 3. Packets 3 and 1 have arrived. What can the receiver conclude?

    Select one answer.

Your practice so far

Check an answer to see its rationale.

Apply it somewhere new

  1. Question 1 of 6A student says, “When my Wi-Fi was down I couldn’t use the Web, so the Web and the Internet are the same.” Which response is correct?

    Select one answer.

  2. Question 2 of 6How did the Internet’s open protocols help it expand?

    Select one answer.

  3. Question 3 of 6In the model network, link C–T fails. Which path from S to T still works?

    Select one answer.

  4. Question 4 of 6A fictional office network has links Hub–W1, Hub–W2, Hub–W3 and Hub–Router, and Router–Internet. Which two are single points of failure for every workstation’s Internet access? Select two.

    Select exactly two answers (0 of 2 selected).

  5. Question 5 of 6Network X connects S to T through one path. Network Y connects S to T through three paths that share no links. If any one link fails, which statement is correct?

    Select one answer.

  6. Question 6 of 6Why does adding redundant paths not guarantee a network will never have an outage?

    Select one answer.

Your practice so far

Check an answer to see its rationale.