IB Computer Science A2.3: Data transmissions

New IB Computer Science syllabus · first exams May 2027 · Theme A · SL & HL (A2.3.4 HL only)

A2.3 is about how data actually gets from one device to another: how devices are addressed, what physical media carry the signal, and how packets find their way across networks.

Key points

A2.3.1 IP addressing

• IPv4: 32-bit, written in dotted decimal (192.168.1.45), with about 4.3 billion addresses. Now nearly exhausted.
• IPv6: 128-bit, written in hexadecimal groups, with about 3.4 × 10³⁸ addresses. IPsec is built in and NAT isn't needed.
• Public vs private:
  • Public addresses are assigned by an ISP and are globally routable.
  • Private addresses (10.x.x.x, 172.16–31.x.x, 192.168.x.x) are used only inside a network.
• Static vs dynamic:
  • Static addresses are fixed, and are used for servers that must always be found at the same address.
  • Dynamic addresses are assigned by DHCP, and are used for laptops and phones.
• NAT: lets many private devices share one public IP. It saves IPv4 addresses and hides internal devices from the internet.

A2.3.2 Transmission media (Compare)

Compare fibre optic, twisted pair and wireless on the eight factors the guide lists:

A2.3.3 Packet switching

1. The data is split into packets.
2. Each packet gets a header with the source and destination IP addresses, a sequence number and a checksum.
3. Packets travel independently and may take different routes. Routers read each header and forward the packet using their routing tables.
4. At the destination, the packets are reassembled in order using the sequence numbers. Only lost or damaged packets are resent.

A2.3.4 Static vs dynamic routing (HL only)

• Static routing: routes are set manually. It's predictable and low overhead, but doesn't adapt to failures and becomes impractical on large networks.
• Dynamic routing: routers use routing protocols to discover routes and update automatically. It adapts to failures and scales well, but uses more CPU and bandwidth and is more complex.

Use the guide's factors: configuration, maintenance, complexity, resource usage, convergence, scalability and network size.

Exam-style question

Explain how packet switching is used to send a large file across the internet. Your answer should include the role of sequence numbers and routers. [5]

Mark scheme

Award [1] for each point, up to [5]:

• The file is split into packets;
• Each packet is given a header with source and destination IP addresses and a sequence number;
• Packets are sent independently and may follow different routes;
• Routers read each header and forward the packet towards its destination;
• The destination device reassembles the packets in the correct order using the sequence numbers;

Common mistakes that cost marks

• Listing media facts without consequences in a "Compare" question.
• Forgetting sequence numbers when explaining how packets are put back together.
• Saying NAT "encrypts" traffic. It translates addresses and hides internal devices; it doesn't encrypt anything.

Frequently asked questions

Which transmission media are assessed?
Fibre optic, twisted pair and wireless only, compared on eight factors: bandwidth, installation complexity, cost, range, interference, attenuation, reliability and security.

Do I need to know specific routing protocols?
No. You need to explain static vs dynamic routing, but specific routing protocols are not required.

Go further

• Practise A2.3 with instant marking on ExamHaus. → examhaus.com
• Full A2.3 presentation and question bank with mark scheme on CS Exam Ready. → csexamready.com/resources/ib-diploma
• IB CS mock exams: Paper 1 & 2, SL & HL. → examhaus.com/mock-exams