How computers talk to each other. Learn how the Internet moves data with protocols, packets, and routing, how networks stay reliable through fault tolerance and redundancy, and how parallel and distributed computing solve big problems faster.
Six ways to master Unit 4 — pick whichever fits how you like to study.
All 3 topics from the College Board CED, in order.
Unit 4 covers Big Idea 4: Computer Systems & Networks — how computers connect and cooperate. It starts with the Internet: a network of networks in which every device has an IP address, and data is broken into packets that travel independently and are reassembled at the destination. Communication follows agreed-upon protocols (such as TCP/IP, HTTP, and DNS), and routers forward packets along paths through the network.
Because parts of a network inevitably fail, the unit covers fault tolerance — the ability of a system to keep working when components break. The Internet achieves this through redundancy: multiple connections and paths mean that if one route goes down, data can travel another way. Finally, parallel and distributed computing speed up large tasks by dividing work across multiple processors (parallel) or multiple networked computers (distributed), with speedup measuring how much faster the parallel solution runs.
This is Big Idea 4, worth 11–15% of the multiple-choice exam. The six computational thinking practices below spiral through this and every big idea: