Unit 4: Computer Systems and Networks
AP Computer Science Principles: 48 practice questions with detailed explanations.
Unit Study Guide
Executive Summary
Devices form networks; the Internet routes packets through them; protocols set the rules; encryption and authentication keep data safe.
Devices and networks
A computing device runs a program. A computing system groups devices together; a network connects them so they can communicate. The Internet is a global network of networks with no single owner. Routers forward traffic; DNS translates domain names to IP addresses.
Fault tolerance and parallelism
Redundancy — multiple paths — makes the Internet fault tolerant: when a link fails, packets reroute. Parallel computing splits tasks across processors at once; distributed computing spreads work across networked machines; sequential runs one task at a time.
Packets and protocols
Messages split into packets that carry data plus addressing; packets can travel different routes and arrive out of order, then are reassembled (TCP handles reliability). Protocols like HTTP and TCP/IP set the rules for communication. HTTPS adds encryption via TLS; HTTP does not.
Security
Encryption scrambles data with a key; symmetric uses one key, public-key uses a pair. Malware (viruses, ransomware) and attacks (phishing, denial of service, man-in-the-middle) threaten systems. Authentication verifies identity; strong passwords and two-factor authentication help.
Exam traps
The Web is not the Internet — it is one service on it. HTTPS means encrypted transit, not a hack-proof site. Redundancy enables fault tolerance; a single path does not.