GCSE · Computer Science · AQA · Spec 8525

Internet layer (TCP/IP)

The layer that decides where your data goes makes no promise that it arrives.

Computer Science · Structure

The four-layer TCP/IP stack

Tap each layer to see what it does. Then ask yourself: which layer makes the delivery promise?

Tap any part of the diagram to see what it does.

Computer Science · The internet layer

A router reads the address, then picks one route

A packet arrives at a router. Choose the destination address it finds and see where a packet goes.

What the destination address says

3 possible routes.

On Packet arrives at a router. 3 branches to choose from.

Tap a destination address the router finds and see where a packet goes.

Computer Science · The internet layer

Who makes the delivery promise?

A message is split into packets, and the packets cross several networks to reach another computer.

Which of these is closest to what you think right now?

The internet layer routes the packets, so it also makes sure they all arrive, and in order.TCP is part of the internet layer, working alongside IP.A router ignores a packet's addresses and sends it wherever it likes.The internet layer addresses and routes each packet. TCP at the transport layer handles reliable, ordered delivery.

How sure are you?

I'm sureI think soI'm guessing

The internet layer routes the packets, so it also makes sure they all arrive, and in order.

This is a common misconception

It's a tempting link: the layer that moves packets must surely look after them. But the internet layer only addresses and routes each packet. It gives no guarantee of arrival or order. Reliable, ordered delivery is handled by TCP at the transport layer.

TCP is part of the internet layer, working alongside IP.

This is a common misconception

Two protocols, two layers. IP works at the internet layer. TCP works at the transport layer, which is the layer directly above it.

A router ignores a packet's addresses and sends it wherever it likes.

This is a common misconception

The addresses are the whole point. IP adds the source and destination IP addresses, and a router uses the destination address to forward a packet.

The internet layer addresses and routes each packet. TCP at the transport layer handles reliable, ordered delivery.

This matches the evidence

That's the division of labour. The internet layer says where each packet goes and makes no promise about arrival or order. The promise belongs to TCP, one layer up.

The layer that does the routing makes no delivery promise. TCP, one layer up, does.

Computer Science · Have a go

Describe it in your own words

Data is sent from a computer on one network to a host on a different network. Describe what the internet layer is responsible for in getting it there. [3 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Adds a source IP address and a destination IP address (to each packet) (1 mark)
Routers read the destination address to forward a packet onwards (1 mark)
Arrival and order are not guaranteed at this layer (1 mark)

Model answer

The internet layer is responsible for addressing and routing. IP gives every packet a source and a destination IP address. Each router along the way reads the destination address to decide where to send it next. This layer makes no promise that a packet turns up or that packets stay in order.

Good answers cover three things: what gets added, what routers do with it, and what is not promised.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

From top to bottom, the four layers are application, transport, internet and link.
The internet layer sits between the transport layer above it and the link layer below it.
Have a goSam is confident: "The internet layer must be at the very bottom. It's the internet, the lowest level of everything!" Which layer is actually below it?
The link layer.
Top to bottom the order is application, transport, internet, link, so the internet layer sits in the middle of the stack, not at the bottom.
The protocol that works at the internet layer is IP, short for Internet Protocol.
IP adds a source IP address and a destination IP address to each packet.
Those addresses are what let a packet be routed across networks to the right host.
A router reads a packet's destination IP address and uses it to forward a packet along one onward route.
Packets with different destination addresses can leave a router by different routes.
Have a goA router holds two packets. One is addressed to a host on network A, the other to a host on network B. Do they have to leave by the same route?
No. Each is forwarded using its own destination address.
The router chooses the onward route from each packet's destination IP address, so different destination addresses can mean different routes.
The internet layer does not guarantee that a packet arrives, or that packets arrive in order.
TCP, at the transport layer, is what provides reliable, ordered delivery.
Have a goTwo packets of one message arrive swapped. Who should have kept them in order: the internet layer or TCP?
TCP, at the transport layer.
The internet layer makes no promise about order. Reliable, ordered delivery is TCP's job one layer up.

The big picture

The TCP/IP model has four layers: application, transport, internet and link. The internet layer sits between transport and link, and its job is addressing and routing. IP adds source and destination IP addresses to each packet, and routers use the destination address to forward it across networks. It does not guarantee arrival or order. TCP at the transport layer provides that.

Key points

1Top to bottom, the four layers are application, transport, internet and link.

2At the internet layer, IP adds source and destination IP addresses to each packet.

3Routers use the destination IP address to forward packets across networks.

4The internet layer gives no guarantee of arrival or order; TCP at the transport layer provides reliable, ordered delivery.

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Worked example

Problem

Three jobs happen when a message crosses between networks: (a) adding the source and destination IP addresses, (b) a router choosing the onward route from the destination address, (c) making sure the packets arrive in the right order. Which layer owns each job?

⚠ Watch out

Assuming the layer that routes packets also makes sure they all arrive in order. It doesn't: the internet layer (IP) addresses and routes, and reliable, ordered delivery is TCP's job at the transport layer.

🧠

Memory hook

IP reads the address. TCP keeps the promise. (Like a sorting office that reads where a parcel is going, but isn't the one who checks the whole delivery turned up.)

✓

Check yourself

Without looking back: which layer sits between transport and link, what two addresses does IP add to each packet, and which layer (not this one) provides reliable, ordered delivery?

Flashcards

(9)

Put the four layers of the TCP/IP model in order, top to bottom.

Application, transport, internet, link.

What does IP stand for, and at which layer does it work?

Internet Protocol. It works at the internet layer.

Which two addresses does IP add to each packet?

A source IP address and a destination IP address.

What does a router read to choose a packet's onward route?

Its destination IP address, not its source address.

Why can two packets leave the same router by different routes?

Each is forwarded using its own destination IP address, and different destinations can need different routes.

Does the internet layer guarantee that packets arrive, or arrive in order?

No. It addresses and routes; it makes no guarantee of arrival or order.

Which layer and protocol provide reliable, ordered delivery?

The transport layer, using TCP.

True or false: TCP works at the internet layer.

False. TCP works at the transport layer; IP works at the internet layer.

In a few words, what is the internet layer's job?

Addressing and routing: getting each packet to the right host across networks by its address.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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The four-layer TCP/IP stack

A router reads the address, then picks one route

Who makes the delivery promise?

Describe it in your own words

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