AP Computer Science Principles Unit 4: Computer Systems and Networks Notes

Introduction

Networks move data through layers of open protocols, dynamically selected paths, and coordinated computing systems. Bandwidth describes capacity, while protocol contracts describe behavior and guarantees.

Trace the service each layer provides: DNS names, IP forwards, TCP or UDP transports, and HTTP supports the Web over Internet infrastructure. Packets can travel independently; redundancy protects only against named failures; and parallel work is limited by dependencies and coordination.

Current exam guidance: Aligned to the Fall 2023 AP CSP framework used in 2026–27. The May 14, 2027 digital exam includes 70 multiple-choice questions in 120 minutes plus two Create-related written-response questions in 60 minutes. The Create components are due April 30, 2027 at 11:59 p.m. ET; current College Board directions always control submission policy.

4.1

The Internet

How can one message travel across many independently operated networks?

Short answer

The Internet joins networks through open protocols: addressed packets follow dynamically selected paths, and endpoint protocols provide only the delivery guarantees their contracts specify.

Know firstAbstraction · Graph paths · Digital data

Separate naming, addressing, routing, and reliable delivery

A computing device can run a program; a computing system combines devices and programs for a purpose; and a network connects devices so they can exchange data. The Internet is a network of networks using standardized open protocols. Its routing is dynamic rather than fixed in advance, and its scalability lets it add devices and networks. Bandwidth measures the maximum data that can be sent in a fixed time, usually in bits per second; it is capacity, not a guarantee that every transfer reaches that rate.

An application data stream can be divided into packets containing a data chunk plus metadata used for routing and reassembly. Internet Protocol supplies addressing and packet-forwarding concepts, while routers choose next hops from current routing information. Packets from one message may follow different paths, arrive out of order, or fail to arrive.

Endpoint and application protocols add different services. TCP supports ordered, reliable delivery; UDP does not supply the same delivery guarantees and can be chosen when an application handles loss or values lower overhead. DNS maps domain names to address information. The World Wide Web is a system of linked pages, programs, and files that uses the Internet, with HTTP as a Web protocol; the Web and Internet are not synonyms.

Packet model trace

Route three packets and reassemble by sequence number

message → packets P1, P2, P3
P1 path: S → N → D
P2 path: S → W → E → D
P3 path: S → N → D
arrival order: P2, P1, P3
endpoint order: P1, P2, P3

Execution stateStep 1 of 5

packet: P1
sequence: 1
path: S-N-D
arrivalPosition: 2

Open complete trace table

Complete trace states
Step	packet	sequence	path	arrivalPosition	endpointAction	reconstructedOrder	layerCheck	result
1	P1	1	S-N-D	2	—	—	—	—
2	P2	2	S-W-E-D	1	—	—	—	—
3	P3	3	S-N-D	3	—	—	—	—
4	—	—	—	—	restore sequence under a reliable-delivery contract	P1, P2, P3	—	—
5	—	—	—	—	—	—	DNS names; IP forwards; TCP orders and recovers; HTTP serves Web resources	roles remain distinct

Output

P1|P2|P3

Packet two arrives first by a different route, followed by packets one and three. Under the modeled reliable-delivery contract, the endpoint restores packet order one, two, three; DNS, IP, TCP, and HTTP still perform different roles.

Analyze a network claim by layer

Identify whether the question concerns devices, bandwidth, naming, routing, transport, or the Web.
Trace each packet independently through the topology and preserve its metadata.
Apply only guarantees stated for IP, TCP, UDP, DNS, or HTTP.
Separate Internet infrastructure from the Web service that runs on it.

Keep these three ideas

Open protocols let the Internet scale across unlike networks.
Packets can take different paths or fail to arrive.
Bandwidth, DNS, IP, TCP or UDP, and HTTP describe different layers or properties.

Computer Systems and Networks