AP CSP Big Idea 4: Computer Systems & Networks | Complete 2025 Study Guide

AP CSP Big Idea 4: Computer Systems & Networks — Complete 2025 Study Guide

Exam Weight: 11–15% • Approximately 8–11 Questions • 2025–2026 AP Exam

What You Will Learn: Big Idea 4 covers how computers communicate across networks. You will understand how the internet uses packets and protocols to deliver data, how redundant connections create fault-tolerant networks, and how parallel and distributed computing increase the power of computation. Network diagram questions and speedup calculations appear regularly on the AP exam.

Table of Contents

How the Internet Works

The internet is a global network of interconnected computers and devices. It uses a system of agreed-upon rules called protocols to allow devices made by different manufacturers to communicate with each other.

Key Characteristics of the Internet

  • The internet is scalable — new devices can join without requiring changes to the existing network
  • Communication is decentralized — there is no single controlling hub; data can travel many different paths
  • The internet uses a layered design where each layer handles a specific function

IP Addresses

Every device connected to the internet has an IP address (Internet Protocol address) that uniquely identifies it on the network. IP addresses work like mailing addresses — they tell routers where to deliver data.

  • IPv4: 32-bit address written as four decimal numbers (e.g., 192.168.1.1); can address about 4 billion devices
  • IPv6: 128-bit address; created because IPv4 addresses were running out; can address a vastly larger number of devices

Bandwidth and Latency

Term What It Measures Analogy
Bandwidth How much data can be transmitted per unit of time (bits per second) Width of a highway — more lanes = more cars at once
Latency The time delay for a signal to travel from sender to receiver How long a single car takes to drive from A to B
AP Tip: High bandwidth does not guarantee low latency. A connection can transfer large amounts of data (high bandwidth) but still have noticeable delays (high latency). Video calls are sensitive to latency; file downloads care more about bandwidth.

Protocols: TCP/IP, HTTP, DNS

A protocol is a set of rules that defines how data is formatted and transmitted between devices. Without protocols, different computers and software could not communicate.

Protocol Full Name What It Does
IP Internet Protocol Assigns addresses to devices and handles routing of packets from source to destination
TCP Transmission Control Protocol Breaks data into packets, ensures all packets arrive, and reassembles them in the correct order
HTTP HyperText Transfer Protocol Governs how web pages are requested and delivered between browsers and web servers
HTTPS HTTP Secure HTTP with encryption; protects data in transit between the browser and the server
DNS Domain Name System Translates human-readable domain names (google.com) into IP addresses (142.250.80.46)
Key Idea: DNS is like a phone book for the internet. When you type a website address, DNS looks up what IP address that name corresponds to, then your request is routed to that address. Without DNS, you would need to memorize IP addresses to visit websites.

Packets and Routing

Data sent over the internet is not transmitted as one continuous stream. Instead, it is broken into small chunks called packets, each of which may travel a different path across the network.

How Packet Switching Works

  1. The sender's device breaks the data into packets
  2. Each packet includes the destination IP address and a sequence number
  3. Routers forward each packet toward the destination, choosing the best available path
  4. Packets may arrive out of order or take different routes
  5. The receiving device reassembles the packets in the correct order using sequence numbers
Example: When you send an email with an attachment, the email is split into dozens or hundreds of packets. Some packets might travel through Chicago while others go through Dallas. All of them arrive at the destination where they are reassembled into the original email. You see only the finished result.

Why Packet Switching is Better Than Circuit Switching

Older telephone networks used circuit switching: a dedicated physical path was reserved for the entire duration of a call. Packet switching is more efficient because:

  • Multiple conversations can share the same physical connections
  • If one path is congested or broken, packets are automatically rerouted
  • Network resources are used only when data is actually being sent

Fault Tolerance and Redundancy

The internet was originally designed to remain functional even if parts of it were destroyed or disabled. This is achieved through redundancy — having multiple paths between any two points.

Redundant Connections

A network is fault tolerant if it can continue functioning when one or more components fail. If the only path between two devices is cut, they lose connectivity. If multiple paths exist, traffic is automatically rerouted around the failure.

Key Idea: Adding redundant connections increases fault tolerance but also increases cost and complexity. The AP exam often presents network diagrams and asks you to identify which device failures disconnect the network, or how many connections can fail before two nodes can no longer communicate.

Analyzing Network Diagrams

When given a network diagram on the AP exam, follow these steps:

  1. Identify all devices (nodes) and connections (edges)
  2. For fault tolerance questions, ask: "If device X is removed, can A still reach B?"
  3. Trace all possible paths between the two nodes in question
  4. If all paths go through a single device, removing that device disconnects the network
Example — Single Point of Failure: In a network where devices A, B, C, and D all connect through a central hub H, removing H disconnects all four devices from each other. H is a single point of failure. Adding direct connections between some devices would eliminate this vulnerability.
AP Tip: Network diagram questions are very predictable. On scratch paper, draw circles for nodes and lines for connections. Then systematically remove the specified device and check if a path still exists between the two named endpoints.

Sequential vs. Parallel Computing

Problems can be solved using different computing strategies. Understanding the trade-offs between sequential and parallel approaches is important for the AP exam.

Sequential Computing

In sequential computing, tasks are performed one at a time in order. Each step must complete before the next one begins. Most basic programs work this way.

Parallel Computing

In parallel computing, a problem is broken into smaller subtasks that can be solved simultaneously by multiple processors working at the same time.

Key Idea: Not all problems can be parallelized. Some tasks depend on the results of previous steps, so they must be performed sequentially. Problems where tasks are independent of each other benefit most from parallel computing.

Calculating Speedup

The AP exam may ask you to calculate how much faster a program runs when parallelized. The key insight is that the total time is limited by the sequential portion that cannot be parallelized.

Speedup Example:

A program has 4 tasks:
Task A: 10 seconds (must run first — sequential)
Task B: 5 seconds (can run in parallel)
Task C: 5 seconds (can run in parallel with B)
Task D: 3 seconds (must run after B and C — sequential)

Sequential time: 10 + 5 + 5 + 3 = 23 seconds
Parallel time: 10 + max(5, 5) + 3 = 18 seconds
Speedup: B and C run simultaneously, saving 5 seconds.
Common Mistake: Students try to parallelize everything. Remember: steps that depend on earlier results (like Task D depending on B and C) still must wait. The sequential bottlenecks determine the minimum possible runtime.

Limits of Parallel Computing

  • The portion of a task that must remain sequential limits the maximum speedup (Amdahl's Law concept)
  • Coordinating parallel tasks adds some overhead
  • More processors do not always mean proportionally faster results

Distributed Computing

Distributed computing uses multiple computers connected across a network to collaboratively solve a problem that would be too large or slow for a single computer.

How Distributed Computing Works

  • A large problem is divided into smaller pieces
  • Each piece is sent to a different computer to process
  • Results are collected and combined
Examples of Distributed Computing:
  • SETI@home: Volunteers donated their computers' idle processing time to analyze radio telescope data searching for signs of extraterrestrial intelligence
  • Folding@home: Distributed protein folding simulations to help researchers understand diseases
  • Cloud computing: Companies rent distributed computing resources from providers like AWS or Google Cloud rather than owning physical servers

Crowdsourcing

Crowdsourcing uses contributions from large numbers of people or computers to accomplish a task. It is related to distributed computing but often involves human effort rather than just processing power.

  • Wikipedia is built through crowdsourcing human knowledge
  • reCAPTCHA uses human responses to train image recognition AI
  • Distributed computing projects crowdsource processor cycles

Key Vocabulary

Term Definition
Protocol A set of rules that governs how data is formatted and transmitted between devices on a network
IP address A unique numerical label assigned to every device on a network; used to route data to the correct destination
DNS Domain Name System; translates human-readable domain names into IP addresses
TCP Transmission Control Protocol; breaks data into packets, ensures all arrive, and reassembles them
HTTP/HTTPS Protocols governing web page requests and delivery; HTTPS adds encryption for security
Packet A small chunk of data sent across a network; includes destination address and sequence number
Routing The process of forwarding packets from source to destination across network devices called routers
Bandwidth The maximum amount of data that can be transmitted per unit of time (measured in bits per second)
Latency The time delay for data to travel from sender to receiver
Fault tolerance The ability of a network to continue functioning when one or more components fail
Redundancy Having multiple paths or components so that failure of one does not disable the system
Parallel computing Breaking a problem into subtasks that are solved simultaneously by multiple processors
Sequential computing Executing tasks one at a time in order, where each step waits for the previous one to complete
Distributed computing Using multiple networked computers to collaborate on solving a large computational problem
Crowdsourcing Obtaining contributions from large numbers of people or devices to accomplish a task

AP Exam-Style Practice Questions

Predict your answer before revealing it.

Question 1

A user types "www.apcsexamprep.com" into a web browser. Which of the following accurately describes what happens before the browser can retrieve the web page?

  • (A) The browser contacts the web server directly using the domain name as its address
  • (B) A DNS lookup translates the domain name into an IP address, which the browser then uses to contact the web server
  • (C) The browser sends the request to all IP addresses on the internet until it finds the correct server
  • (D) HTTP encrypts the domain name and sends it to the server without requiring an IP address
Show Answer & Explanation

Answer: B

DNS (Domain Name System) is the internet's address translation service. When you type a domain name, your computer first queries a DNS server to get the corresponding IP address. The browser then uses that IP address to send the HTTP request to the correct server. Eliminate (A): domain names are not valid network addresses — IP addresses are. Eliminate (C): broadcasting to all addresses would be massively inefficient and is not how routing works. Eliminate (D): HTTP does not perform DNS lookups, and encryption requires HTTPS not plain HTTP.

Question 2 — Network Diagram

In a network, devices A, B, C, D, and E are connected as follows: A connects to B and C. B connects to A and D. C connects to A and D. D connects to B, C, and E. If device B fails, can A still communicate with E?

  • (A) No, because B is the only connection between A and the rest of the network
  • (B) Yes, because A can reach E through the path A → C → D → E
  • (C) Yes, because A connects directly to E through D
  • (D) No, because removing any device from a network always disconnects it
Show Answer & Explanation

Answer: B

Trace the available paths with B removed. A connects to C. C connects to D. D connects to E. Therefore A → C → D → E is a valid path. The network has redundant routing, so B's failure does not disconnect A from E. Eliminate (A): A has two connections (B and C); losing B still leaves C. Eliminate (C): A does not connect directly to D in the described network. Eliminate (D): this is false — redundant networks are specifically designed to survive individual node failures.

Question 3 — I, II, and III format

A programmer is optimizing a data processing program. The program consists of three tasks. Task X takes 20 seconds and must run first. Task Y takes 8 seconds and Task Z takes 6 seconds; both can run simultaneously after Task X finishes. Which of the following statements about parallelizing Tasks Y and Z are true?

I. Running Y and Z in parallel reduces total runtime compared to running them sequentially.
II. The total runtime when Y and Z run in parallel is 28 seconds.
III. The speedup from parallelization is unlimited as long as more processors are added.

  • (A) I only
  • (B) I and II only
  • (C) II and III only
  • (D) I, II, and III
Show Answer & Explanation

Answer: B — I and II only

Statement I is true: sequential runtime = 20 + 8 + 6 = 34 seconds. Parallel runtime = 20 + max(8, 6) = 20 + 8 = 28 seconds. Parallel is faster. Statement II is true: as calculated, 20 + 8 = 28 seconds (Y is the bottleneck). Statement III is false: speedup is limited by the sequential portion (Task X's 20 seconds). No amount of additional processors can reduce the time below 20 seconds, because Task X must complete before any parallelization can begin. This is the core concept of Amdahl's Law. Since III is false, eliminate (C) and (D).

Question 4 — Spot the Error

A student describes how the internet transmits data: "When you send a large file, a direct dedicated connection is established between your computer and the destination, and all data flows through that single reserved path until the transfer is complete." Which part of this description is incorrect?

  • (A) Large files cannot be transmitted over the internet; they must be stored on physical drives
  • (B) The internet uses packet switching, not circuit switching; data is broken into packets that may travel different paths and arrive out of order
  • (C) The description is correct; the internet does reserve dedicated connections for individual file transfers
  • (D) The description is only incorrect because it does not mention that HTTPS must be used for file transfers
Show Answer & Explanation

Answer: B

The student's description accurately describes circuit switching (used in traditional telephone networks), not how the internet works. The internet uses packet switching: data is divided into packets, each packet is routed independently, and packets may arrive out of order before being reassembled. There is no dedicated reserved path. This design is what makes the internet fault tolerant and efficient. Eliminate (A): completely false. Eliminate (C): this incorrectly validates the student's wrong description. Eliminate (D): HTTPS vs HTTP is about security, not routing architecture.

Question 5

A research team needs to analyze 10 petabytes of astronomical data to search for exoplanets. The analysis of each segment of data is independent of all other segments. Which computing approach would most effectively reduce the total analysis time?

  • (A) Sequential computing on a single very fast processor, because a single system avoids the overhead of coordinating multiple computers
  • (B) Sequential computing with high-bandwidth storage, because reading data faster is the primary bottleneck
  • (C) Distributed computing across many machines, because the independent segments can be analyzed simultaneously, drastically reducing total time
  • (D) Parallel computing on a single machine with multiple processors, because distributed computing requires sending data across a network which is slow
Show Answer & Explanation

Answer: C

The key detail is that each segment of data is independent — this means the work is perfectly suited for parallelization across many machines. Distributing 10 petabytes across thousands of computers means all segments are analyzed simultaneously, which dramatically reduces total time. Eliminate (A): a single processor, no matter how fast, must process segments one at a time. Eliminate (B): storage speed is not the stated bottleneck. Eliminate (D): while network overhead is real, it is trivial compared to the massive time savings from analyzing all segments simultaneously. Projects like SETI@home and Folding@home use exactly this approach for astronomical-scale datasets.

Continue Studying: Big Idea 4 is 11–15% of the exam. Focus on network diagrams (trace paths, find single points of failure), parallel computing speedup calculations, and understanding the difference between sequential, parallel, and distributed approaches. These are the highest-frequency BI4 question types.
← Big Idea 3 Big Idea 5: Impact of Computing →

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