GCSE · Computer Science · AQA · Spec 8525
IP (Internet Protocol)
Your message is about to be chopped up and sent across the internet. Nobody steers the pieces, and they may arrive in the wrong order.
Computer Science · Networks
Follow the packets
The message HELLO! is split into two packets. The boxes list the places a packet can pass through. They are not one fixed route, so read each note to see where each packet goes.
Packet 1 leaves Host A before packet 2. Which one do you think reaches Host B first? Step through and find out.
Message: HELLO! split into packet 1 (HEL) and packet 2 (LO!)
Output
Step 1: Host A splits the message into packets. Each one is labelled with two addresses: where it came from and where it is going. Packet 1 goes first.
Step through the trace — every value is the lesson’s, not run by the page.
Computer Science · Networks
Whose job is it?
Which protocol does each job?
Still to sort
IP (internet layer) (0)
Gets packets from device to device across networks.
Where the line is: IP addresses and forwards. It does not promise that anything arrives or arrives in order.
TCP (transport layer) (0)
Provides reliable, ordered delivery.
Where the line is: TCP is the one that makes the promise IP won't make.
IP and TCP work as a team. Pick a job, then pick who does it.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Nobody plans the route. Every router just passes each packet one hop closer.
What you need to know
- IP stands for Internet Protocol. It gets packets from one device to another across networks.
- Every device has an IP address, and that address identifies the device.
- Each packet carries two addresses: the source IP address and the destination IP address.
Have a goYou send a photo from your laptop to a friend's phone. Which two addresses does each packet carry, and which one says where it is heading?
The source IP address (your laptop) and the destination IP address (your friend's phone). The destination says where it is heading.
Source is where a packet came from, destination is where it is going. It's easy to swap them.
- A router reads a packet's destination IP address and forwards it one hop closer.
- A hop is one step of the journey, such as from one router to the next. Nobody plans the whole route.
Have a goQuiet Tuesday. You are a router and a packet lands in front of you. What do you read, and how far do you send it?
Read the destination IP address, then forward it one hop closer.
A router only decides the next hop. It doesn't plan a packet's whole journey.
- Packets of one message may take different routes, so they can arrive in a different order.
- IP guarantees neither delivery nor order.
- That's why IP is used with TCP, which provides reliable, ordered delivery.
Have a goFinish the sentence: IP doesn't guarantee ___ or ___, so it is used with ___.
Delivery, order, TCP.
TCP provides reliable, ordered delivery, so the two protocols together cover what IP leaves out.
- IP works at the internet layer. TCP works at the transport layer.
The big picture
IP gets packets from one device to another across networks. Every packet carries a source and a destination IP address, and each router reads the destination and forwards each packet one hop at a time. Packets of one message may take different routes, and IP guarantees neither delivery nor order, which is why it is used with TCP.
Key points
Worked example
Problem
A message is split into four packets, sent in the order 1, 2, 3, 4. Host B receives all four, but in the order 2, 4, 1, 3. Has IP failed? Explain, and say what is used with IP so the message can be put back together.
⚠ Watch out
Handing TCP's jobs to IP. IP addresses packets and gets them forwarded hop by hop; making sure everything arrives, in order, is not on its list.
Memory hook
IP is the address label, not the promise. Label on, hop by hop, no guarantees. The promise comes from TCP.
Check yourself
A packet reaches a router on its way across the internet. In your own words: what does the router read, what does it do with it, and does it plan the rest of the route?
Flashcards
(10)What do the letters IP stand for?
What does an IP address do?
Which two addresses does each packet carry?
What does a router do with a packet's destination IP address?
What is a hop?
Who plans a packet's whole route in advance?
Must every packet of one message take the same route?
Does IP guarantee delivery or order?
Which protocol provides reliable, ordered delivery alongside IP?
At which layers do IP and TCP work?
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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