How does the internet work, and what is the TCP/IP protocol stack?
Understand the structure of the internet, packet switching, the TCP/IP four-layer model, IP addressing, DNS, routers and gateways, and how data is routed across networks.
A focused answer to AQA A-Level Computer Science 4.8.4, covering the structure of the internet, packet switching, the TCP/IP four-layer model, IP addressing, DNS, routers and gateways, and how data is routed across networks.
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What this dot point is asking
AQA wants you to describe the internet as a network of networks, explain packet switching, describe the four layers of the TCP/IP model, explain IP addressing and DNS, and describe the role of routers and gateways in routing data.
The internet and packet switching
Packet switching contrasts with circuit switching, where a dedicated path is reserved for the whole conversation (as in a traditional phone call). By splitting data into independently addressed packets, the internet lets many communications share the same links, so capacity is used efficiently, and it can reroute packets around a broken link without dropping the connection. This is why the internet is robust even when individual routers or cables fail, an idea that connects to the resilience of the network's overall structure.
The TCP/IP model
The layered design is itself an example of abstraction: each layer provides a service to the one above and relies on the one below, without either needing to know the other's internal detail. As data is sent it passes down the stack, each layer adding its own header (encapsulation); at the receiving end it passes up the stack, each layer stripping its header. This separation is why a browser using HTTP does not need to know whether the link layer is Ethernet or Wi-Fi, and why new application protocols can be added without changing how packets are routed.
IP addressing, DNS, routers and gateways
DNS works like a distributed directory: when you type a domain name, your computer queries DNS servers to look up the matching IP address before any connection is made, much like looking up a contact's number before calling. Routers then use those IP addresses to forward each packet hop by hop towards its destination, while a gateway is needed only where two networks speak different protocols and a translation is required. Distinguishing the router (forwards between networks) from the gateway (translates between different protocols) is a common exam discriminator.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksExplain how packet switching is used to send a large file across the internet, and give two advantages of packet switching over sending the file as one continuous stream.Show worked answer →
The file is divided into packets, each carrying the source and destination IP addresses and a sequence number. Each packet is routed independently across the network of routers, possibly taking different paths, and the packets are reassembled into the correct order at the destination using their sequence numbers, with any lost packets re-requested.
Two advantages: first, efficient use of the network, because packets from many communications can share the same links rather than tying up a dedicated line; second, resilience, because if a link or router fails, packets can be routed around the failure by an alternative path rather than the whole transfer failing.
Markers reward dividing into addressed, sequenced packets routed independently and reassembled, plus two valid advantages (efficiency through sharing and resilience through rerouting).
AQA 20214 marksDescribe the role of the transport layer and the network (internet) layer in the TCP/IP model, and explain the purpose of DNS.Show worked answer →
The transport layer uses TCP to split the data into packets, number them with sequence numbers, and ensure reliable, in-order delivery, requesting retransmission of any packets that are lost or corrupted. The network (internet) layer uses IP to add the source and destination IP addresses to each packet and to route the packets across the network towards the destination.
DNS (the Domain Name System) translates human-readable domain names such as example.com into the numeric IP addresses that machines use to locate each other, so users can type memorable names rather than IP addresses.
Markers reward TCP at the transport layer handling packets and reliable delivery, IP at the network layer handling addressing and routing, and DNS as the name-to-IP-address translator.
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Sources & how we know this
- AQA A-level Computer Science (7517) specification — AQA (2015)