AP CSP Big Idea 4 Study Guide: Computer Systems, Networks, the Internet, and Security

Computer system

A collection of hardware and software components that work together to take input, process it, store it, and produce output.

Hardware

The physical parts of a computing system (e.g., CPU, memory, network card, cables, routers, servers).

Software

Programs and instructions that run on hardware (e.g., operating systems, browsers, apps).

System thinking

Analyzing how multiple components and layers work together as a whole system rather than focusing on one device in isolation.

Network

A group of interconnected computing devices that can send data to each other.

Resource sharing

A benefit of networks where devices share access to resources like printers, files, services, or databases.

Internet

A global, decentralized network that connects many separate networks using shared rules (protocols).

Network of networks

A description of the Internet: many different networks, owned by different organizations, interconnected to communicate.

Decentralized network

A network structure with no single central computer or company controlling all communication; many independent networks cooperate via standards.

Physical infrastructure

The real hardware the Internet relies on, such as copper wires, fiber-optic cables, radio transmission equipment, routers, and servers.

LAN (Local Area Network)

A smaller network in a limited area such as a home, school, or office.

WAN (Wide Area Network)

A network that covers a large geographic area; the Internet is the largest example.

Wired connection (e.g., Ethernet)

A network connection using physical cables, typically providing stable, high-speed links.

Wireless connection (e.g., Wi‑Fi/cellular)

A network connection using radio waves, trading some reliability for mobility and convenience.

Wi‑Fi

A local wireless method of connecting to a network; it is not the same thing as the Internet.

Bit

A basic unit of data represented as a 0 or 1; computers store and transmit data as bits.

Packet

A small chunk of data sent over a network, rather than sending one large file as a single piece.

Payload

The portion of a packet that contains the actual data being sent (e.g., part of a photo or message).

Header (metadata)

Packet information used for delivery and reassembly, such as destination/source addressing and ordering info.

Packet switching

An Internet method where data is split into packets, packets may take different routes, and the destination reassembles them.

IP address

A numeric address that identifies a device (or network interface) so packets can be delivered across the Internet.

DNS (Domain Name System)

A distributed naming system that translates human-readable domain names into IP addresses.

DNS caching

Temporarily storing DNS lookup results to speed up future requests, which can also delay how quickly updates spread.

Packet loss

When some packets go missing due to congestion, interference, or hardware problems, requiring retransmission or causing quality drops.

Protocol

A set of rules that specify how data is formatted, transmitted, and received so different systems can communicate.

Open standards

Publicly available protocol specifications that anyone can implement, enabling interoperability and innovation.

Layering

Organizing network communication into layers where each layer provides services to the layer above and relies on the layer below.

IP (Internet Protocol)

A protocol responsible for addressing and routing packets across networks; it is best-effort and does not guarantee reliable delivery.

TCP (Transmission Control Protocol)

A transport protocol that provides reliable, ordered delivery by tracking packets, retransmitting missing ones, and reordering as needed.

UDP (User Datagram Protocol)

A transport protocol with less overhead than TCP that generally does not ensure reliable delivery, often reducing delay for real-time uses.

HTTP (HyperText Transfer Protocol)

A protocol used to request and transfer web resources such as web pages and images.

HTTPS

HTTP used with encryption (commonly via TLS) to protect data in transit between browser and server.

TLS (Transport Layer Security)

The encryption/authentication technology commonly used by HTTPS to provide confidentiality and integrity in transit.

Router

A network device that forwards packets toward their destinations across multiple networks.

Routing

The step-by-step process of forwarding packets from sender to receiver through multiple routers and networks.

Redundancy

Having extra components (e.g., links, servers, copies of data) so the system can keep working if something fails.

Fault tolerance

A system’s ability to keep operating even when some components fail, often enabled by packet switching and redundancy.

Bandwidth

The maximum amount of data that can be transferred over a connection per unit time (capacity), measured in bits per second.

Latency

The time delay for data to travel from source to destination; high latency can hurt real-time interactions.

Throughput

The actual rate at which data is successfully delivered, affected by congestion, packet loss, retransmissions, and overhead.

Congestion

When more data is sent over a network link than it can handle efficiently, increasing delay and possibly packet loss.

Phishing

An attack that tricks users into revealing sensitive information by pretending to be a trusted entity.

Distributed Denial-of-Service (DDoS) attack

A denial-of-service attack launched from many machines at once to overwhelm a service or network and reduce availability.

Cloud computing

Delivering computing services (servers, storage, databases, software) over a network, relying on large-scale distributed systems.

Scalability

The ability of a system or network to handle growing amounts of work efficiently, often by adding resources with minimal interruption.

Parallel computing

Using multiple processors (or cores) to work on different parts of a task simultaneously; total time is limited by sequential parts and the slowest parallel task.

Distributed computing

Spreading computation and storage across multiple networked machines to handle problems too large for one device and to improve scalability/fault tolerance.

Confidentiality

A security goal focused on keeping data secret from unauthorized parties (often supported by encryption).

Integrity

A security goal focused on ensuring data is not altered without detection while in transit or storage.

Availability

A security goal focused on keeping systems and data accessible to authorized users (threatened by outages and DoS/DDoS attacks).

Authentication

Verifying identity (user or server), such as passwords/MFA for users or certificates for servers, to reduce impersonation risk.