Chapter 2: Core Concepts

Chapter 2: Core Concepts

Chapter 2: Core Concepts

Key Vocabulary

English / 英文中文Simple Meaning / 簡單意思
layer層one level in a system
subnet子網路smaller part of a network
infrastructure基礎設施basic systems and equipment
（第二章：核心概念）

Understand principles, relationships, cause-and-effect, classification — not just memorize terms. Chapter 1 gave definitions; this chapter explains why, how, what's different, and when to use it. （理解原理、關係、因果、分類，不是死背名詞）

中文快速導讀

本章只需要先掌握 4 個核心問題：

  1. OSI 七層： 資料由上而下包裝、由下而上拆開；Transport 最常考的是 segmentation（資料分割）。
  2. Subnet Mask（子網路遮罩）： 32 − 網路位元 = 主機位元，可用主機數是 2ⁿ − 2。
  3. Cloud Service Models（雲端服務模式）： IaaS 自己管理最多，SaaS 自己管理最少；題目先問「誰安裝 OS、誰管理程式」。
  4. Number Systems（進位制）： 每 4 個 binary bits（2 進位元）對應 1 個 hexadecimal digit（16 進位數字）。

中文讀題提示：why 問原因、how 問做法、which layer 問層級、maximum 問最多數量。先圈出這些關鍵字，再找答案。

Concept 1: OSI 7-Layer Model — why layers?（為什麼要分層）

One-sentence concept（一句話概念）: Network communication is split into 7 independent layers; data gets wrapped layer by layer going down, and unwrapped layer by layer going up — like nested envelopes.

Visual（視覺化）

7 Application　HTTP／FTP／Email 6 Presentation　SSL／JPEG（format & encryption） 5 Session　NetBIOS／RPC（establishes dialogue） 4 Transport　TCP／UDP（segmentation） 3 Network　IP／ICMP（addressing & routing） 2 Data Link　Ethernet／ARP（MAC addressing） 1 Physical　cable／fiber／Wi-Fi（bit stream） Sender: Application → wraps down → Physical sends out Receiver: Physical → unwraps up → back to Application

Simple explanation（簡單說明）: Layering keeps each layer independent, flexible, and decoupled（各層相互獨立、完全解耦）. Switching a site to HTTPS only touches the Presentation layer — the physical cable never changes. The Transport layer's job of "segmentation"（資料分割）is the single most-tested action in past papers.

Concrete example（具體例子）: Opening a webpage on your phone — Application (HTTP) sends the request → Transport (TCP) splits it into numbered segments → Network (IP) decides the route → Data Link (Ethernet/Wi-Fi) frames it → Physical actually transmits the signal. The server reverses the process on receipt.

Student practice（學生練習）: If a site switches from HTTP to HTTPS, which layer is mainly affected?

Recall question（回想問題，先蓋住上面內容再答）: What's the bottom-to-top order of the 7 layers? Which layer does "segmentation"?

Concept 2: Subnet Mask Calculation — why 2ⁿ − 2?（為什麼要算 2ⁿ − 2）

One-sentence concept: A subnet mask splits an IP address into a network portion and a host portion; the number of host bits determines the maximum devices the subnet can hold.

Visual（視覺化）

Problem: fit 60 devices — minimum host bits?（容納 60 台裝置） find smallest n 2ⁿ−2 ≥ 60 → n=6 host bits = 6（主機位） network bits=32−6=26 255.255.255.192（/26）

Simple explanation: The "−2" removes the network address itself and the broadcast address — neither can be assigned to a host. A larger n gives more usable hosts but fewer subnets — that's the network-bits vs host-bits trade-off.

Concrete example: Fit 200 devices → 2ⁿ−2 ≥ 200 → n=8 (2⁸−2=254) → network bits=32−8=24 → mask 255.255.255.0 (/24).

Student practice: Fit 10 devices — minimum host bits, and what mask does that give?

Recall question: What exactly does the "−2" in the formula subtract?

Concept 3: Three Cloud Service Models — who manages what?（誰管理什麼）

One-sentence concept: IaaS / PaaS / SaaS differ in how much of the stack the user has to manage themselves — the less you manage, the more convenient but the less flexible.

Visual（視覺化）

You manage（自己管理） Vendor manages（廠商代管） IaaS You: OS, apps, data （作業系統／應用／資料） Vendor: hardware, network e.g. Alibaba Cloud ECS PaaS You: apps, data （應用程式、資料） Vendor: OS, hardware, network e.g. dev platforms SaaS You: only the data you type in（僅輸入資料） Vendor: everything e.g. Google Docs

Simple explanation: Think of it as one pyramid at three heights — IaaS only rents hardware, PaaS skips even the OS, SaaS is ready to use. When in doubt, ask: "does the user install an OS? deploy their own code?"

Concrete example: Alibaba Cloud ECS and Tencent Cloud CVM are IaaS (rent a VM, install your own OS); Google Docs and Microsoft 365 are SaaS (open a browser and go).

Student practice: A school renting a VM and installing Linux themselves to run programs — which model is that?

Recall question: What exactly does the user manage themselves under IaaS vs PaaS vs SaaS?

Concept 4: How Number Systems Relate to Each Other（數字系統轉換的關係）

One-sentence concept: Binary, octal, decimal, and hex are just different "bookkeeping" for the same number — binary and hex are the closest pair (every 4 bits = exactly 1 hex digit).

Visual（視覺化）

Binary 1111 Decimal 15 Octal 17 Hex F Every 4 binary bits = 1 hex digit（例：1111 = F） Every 3 binary bits = 1 octal digit（例：001 111 = 17） Decimal is the human-friendly middle step for conversions

Simple explanation: Don't memorize every conversion table — understand that binary groups of 4 bits become hex, and groups of 3 bits become octal. That lets you convert anything instantly.

Concrete example: Decimal 140 to binary: 140 = 128+8+4 = 10001100; to hex, split into two 4-bit groups (1000 / 1100) = 8C.

Student practice: Convert decimal 27 to binary, then to hex.

Recall question: How many binary bits correspond to one hex digit? How many to one octal digit?

30-Second Review（30 秒複習區）

Close your eyes and answer: the OSI 7-layer mnemonic, what "−2" subtracts in the subnet formula, what each of IaaS/PaaS/SaaS manages, and how binary maps to hex. If you can answer all of them, you've passed — otherwise, revisit the matching diagram.

The traps and harder discrimination questions for these concepts live in 04_易錯陷阱與高難辨識_202609181656.md.

Classification Overview (extension)（分類總覽）

Source（來源）: NotebookLM's auto-generated mind map covering all 14 ITE7 chapters + 17 ITN modules — a classification-first pass to check for gaps.

  • Networking overview（電腦網路概述）: network types (WAN/MAN/LAN), performance metrics (throughput/latency/RTT), layering principles
  • Network layer architecture（網路層級架構）: Physical (bit transmission), Data Link (framing, MAC addressing), Network (IP/routing/NAT), Transport (TCP/UDP, three-way handshake, sliding window), Application (DNS/HTTP/DHCP/FTP)
  • PC hardware（個人電腦硬體）: safety (ESD, anti-static wristband), internal components (PSU, motherboard chipset, CPU package, RAM), storage (HDD/SSD/optical), input/output devices
  • Windows install & management（Windows 安裝與管理）: install prep (MBR/GPT, NTFS/FAT32), system tools (Task Manager, Disk Management), advanced (Registry, Group Policy), preventive maintenance (restore points, backups)
  • Other OS & security（其他作業系統與安全性）: mobile OS (Android/iOS), Linux/macOS (CLI commands), the 6-step troubleshooting method (identify → hypothesize cause → test → plan & implement → verify & prevent → document)

These five categories cover almost everything in Chapter 1's quick-reference tables — use this to spot-check for gaps before the exam.

Links to this page:

  1. IT Knowledge Quiz Competition — Full Review Overview
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