Quick Review

The Seven Layers at a Glance

Data unit by layer: bits (1), frames (2), packets (3), segments and datagrams (4), data or messages (5 to 7).

Part One: Multiple Choice

1. Which layer moves bits as electricity, light, or radio waves? a) Layer 1 b) Layer 2 c) Layer 3 d) Layer 4

2. What kind of address does Layer 2 use? a) IP address b) MAC address c) Port number d) Web address

3. What is the job of a router? a) Strengthen a weak signal b) Deliver frames inside one local network c) Forward packets between different networks d) Encrypt web pages

4. Which protocol resends data that goes missing? a) UDP b) TCP c) DNS d) IP

5. What does a port number identify? a) Which network a device is on b) Which cable a signal travels on c) Which conversation on a device data belongs to d) Which country a server is in

6. Which layer would encrypt a web page before it is sent? a) Layer 2 b) Layer 3 c) Layer 6 d) Layer 7

7. What is encapsulation? a) Removing headers as data travels up the layers b) Adding a header at each layer as data travels down c) Compressing a file to make it smaller d) Choosing between TCP and UDP

Part Two: Matching

Match each item on the left with the correct layer on the right. Each layer is used once.

Part Three: Scenarios

For each situation, name the layer most likely involved.

1. A network cable has been chewed by a pet and the connection has dropped completely.

2. A computer has an IP address on the wrong network and cannot reach the internet, though it can see other local devices.

3. A web server is running and reachable, but a firewall is blocking the specific port it listens on.

4. A page loads with strange symbols where the letters should be, because the browser and server disagree about the text encoding.

5. A banking website signs a user out after ten minutes of inactivity.

6. A frame arrives damaged, fails its check value, and is discarded by the receiving device.

7. A user types a web address and gets an error saying the name could not be found, although the connection itself is working.

Answer Key

Part One: Multiple Choice

1. a) Layer 1. The Physical layer turns bits into signals and carries them.
2. b) MAC address. Layer 2 delivers frames to hardware addresses on the local network.
3. c) Forward packets between different networks. Strengthening signals is Layer 1; local frame delivery is Layer 2.
4. b) TCP. UDP sends without acknowledgement and never resends.
5. c) Which conversation on a device data belongs to. This is how a device sorts data between programs.
6. c) Layer 6. Encryption is a presentation-style transformation, though TLS is placed differently by different sources.
7. b) Adding a header at each layer as data travels down. Removing them on the way up is decapsulation.

Part Two: Matching

1. Switch — B, Layer 2. It forwards frames using MAC addresses.
2. IP address — C, Layer 3. It identifies a network and a device on it.
3. Fiber-optic cable — A, Layer 1. It carries bits as pulses of light.
4. HTTPS — G, Layer 7. It is the protocol a browser uses to request pages.
5. Staying signed in — E, Layer 5. This is an ongoing session.
6. UTF-8 — F, Layer 6. It is the agreement about what bytes mean as text.
7. Port 443 — D, Layer 4. Port numbers belong to the Transport layer.

Part Three: Scenarios

1. Layer 1. Damaged physical medium.
2. Layer 3. Addressing and routing between networks.
3. Layer 4. The port is a Transport-layer identifier.
4. Layer 6. An encoding mismatch, not a delivery problem.
5. Layer 5. A session timeout.
6. Layer 2. Frame error detection on the local network.
7. Layer 7. DNS is an application-layer service.

If You Got Some Wrong

Do not re-read the whole book. Go back to the one chapter the question came from, read the ELI Explains section first, then read What This Layer Does. Most confusion at this stage is not about detail. It is about which job belongs to which layer, and the two-sentence version usually fixes it faster than the full explanation.

The Shortest Possible Summary

Layer 1 moves signals. Layer 2 delivers them to the right device nearby. Layer 3 gets them to the right network anywhere. Layer 4 gets them to the right program and decides how carefully. Layer 5 keeps track of the conversation. Layer 6 puts the data in a usable form. Layer 7 is where the asking and answering happens.

Seven jobs, one delivery, over before you notice.