11 Repetition
The content of the course is complete; this chapter and the next consolidate it. Lecture 11 revisits the key points of Modules 1–10 - the big ideas, not every detail - interleaved with quick-check questions in exactly the exam’s style: Four options, one correct. Use this chapter as a revision map: Read a block, try its quick check, and wherever something feels fuzzy, go back to that chapter - the point of a round-up is to find the gaps while there is still time to close them. The goal for the week, in the lecture’s own words: Leave able to trace one click from keypress to rendered page, naming every layer on the way. The course falls into five blocks for review.
11.1 Block 1: Data and numbers
Every positional system has a base and digits with place value: Decimal (base 10), binary (base 2), octal (base 8), hexadecimal (base 16, digits 0–9 and A–F). Computers store everything in binary; hex is the compact human notation for groups of four bits (Chapter 2). The core skill is moving between decimal and binary with the place values 128–64–32–16–8–4–2–1: Binary 0001\,1010 = 16+8+2 = 26; decimal 45 = 32+8+4+1 = 0010\,1101. One byte covers 0–255 - which is exactly why an IPv4 octet stops at 255 (Chapter 9, closing the loop). Binary addition works like decimal with carries at two (1+1 = 10: Write 0, carry 1); multiplication is shift-and-add; and negatives use two’s complement - invert all bits, add 1 - which avoids a double zero and turns subtraction a-b into the addition a + (\text{two's complement of } b), so the CPU needs only an adder (Definition 2.2, Example 4.1). For text: ASCII gives 7 bits and 128 characters - English only; Unicode gives every character a code point (Å is U+00C5); UTF-8 encodes those code points at variable length - one byte for ASCII, more for the rest - and its backward compatibility with ASCII is why it became the web’s default (Chapter 3).
Quick check. What is binary 1011 in decimal? (A) 9 (B) 11 (C) 13 (D) 7. - Answer: (B), since 8+2+1 = 11.
11.2 Block 2: Logic and the computer
Gates compute the Boolean basics - AND (both), OR (at least one), XOR (exactly one: The inputs differ), NOT (invert) (Definition 4.1). Combine them and you can do arithmetic: The full adder takes bits A, B and a carry-in, produces a sum bit and a carry-out, and chained adders add whole binary numbers - the bridge from logic to computing (Example 4.1). Wire enough together and you have the Von Neumann machine: One memory for code and data, a CPU repeating fetch–decode–execute (in full, the HIDAS cycle of Definition 4.2) one instruction at a time, and a memory hierarchy - registers, cache, RAM, disk - trading speed for size, with hot data kept near the CPU (Chapter 4). Raw hardware is unusable alone; the operating system makes it a computer: Abstraction above (files, windows, the shell), resource management below (processes and the scheduler sharing the CPU; virtual memory; the file system), with user mode versus kernel mode keeping one misbehaving program from taking down the machine (Chapter 5).
Quick check. Which gate outputs 1 only when its two inputs differ? (A) AND (B) OR (C) XOR (D) NAND. - Answer: (C) - 1 for 01 and 10, 0 for 00 and 11.
11.3 Block 3: The internet and the application layer
The internet joins independent networks through shared protocols (TCP/IP); two reference models describe the stack - OSI with seven layers, TCP/IP with five - and ours is the five: Application, transport, network, link, physical, each serving the layer above and talking to its peer opposite (Definition 6.3). Data moves down by encapsulation: Transport adds ports and the unit is a segment; network adds IP addresses - a datagram; link adds MACs - a frame; the receiver unwraps in reverse, and the unit’s name tells you the layer (Definition 6.4). At the top, DNS is the first step of almost every connection: A distributed hierarchy - root, TLD, authoritative - turning www.kristiania.no into an address, with answers cached under a TTL (Definition 7.2); HTTP fetches the web in request/response pairs with methods, status codes and cookies; SMTP pushes mail towards the recipient while POP3/IMAP pull it back; and the plaintext protocols have encrypted successors - HTTPS over TLS, SFTP over SSH - because anything unencrypted is readable on the path (Chapters 6–7).
11.4 Block 4: Transport and network
Transport connects processes via ports; IP plus port is a socket (Definition 8.1). UDP is connectionless and lean - best for live media and DNS, where a glitch beats a stall; TCP opens a connection with the three-way handshake and delivers a reliable, ordered byte stream through ACKs, retransmission, flow control and congestion control (Chapter 8). Beneath it, the network layer routes packets host to host, hop by hop, by forwarding tables; IPv4 addresses are 32 bits, IPv6’s are 128; and CIDR’s /n splits every address into network and host part - host 10.21.26.184 with mask /22: Third octet 26 AND 252 = 24, so network 10.21.24.0/22, with 32-22 = 10 host bits giving 1024 addresses, 1022 usable (Example 9.1). Three helpers make IP networks livable: DHCP hands your machine an address (plus gateway and DNS) automatically; ICMP carries errors and the echo behind ping, while its “time exceeded” powers traceroute; and NAT lets a whole home share one public address through a port-based translation table - a big reason IPv4 lasted so long (Chapter 9).
Quick check. Which transport protocol is connectionless and best for live video? (A) TCP (B) UDP (C) IP (D) ARP. - Answer: (B) - no handshake, tolerates loss; a glitch beats a stall.
11.5 Block 5: The link layer, and the whole journey
The link layer runs in the adapter and carries a frame across one hop, wrapped with a CRC check. MAC addresses are flat, 48-bit and per-adapter - local scope only - so IP stays end-to-end while the MAC changes on every hop; ARP maps a neighbour’s IP to its MAC by broadcast-and-reply (Definition 10.1 and Definition 10.2). Sharing a medium needs an access rule: Wired Ethernet historically detected collisions (CSMA/CD) and now avoids them structurally with self-learning switches that isolate each port; Wi-Fi must avoid collisions (CSMA/CA), since a radio cannot detect its own. Keep the boxes straight: A switch forwards within a LAN by MAC; a router forwards between networks by IP (Chapter 10). And the whole course compresses into one action: You type a URL and DNS finds the server’s IP (Chapters 6–7); TCP opens a reliable connection (Chapter 8); IP routes the packets hop by hop (Chapter 9); ARP and the link layer carry each frame across each hop (Chapter 10); HTTP returns the page and the browser renders it - all of it as bits under conventions (Chapters 1–3). If you can narrate that trace unaided, you are ready.
Quick check. ARP is used to find a node’s ＿＿ when you know its ＿＿. (A) IP / MAC (B) MAC / IP (C) port / IP (D) name / IP. - Answer: (B) - ARP maps a known IP to the matching hardware MAC.
Interactive checks
A round of quick interactive drills to close the revision - instant feedback, all in your browser, nothing recorded. Every task has a show-solution button; clicking it straight away is no fun.
11.6 The big picture - and one closing note
If you remember five things from the semester, the lecture nominates these: Everything is bits - numbers, text, logic, all binary; gates build computers, and the OS makes them usable; the internet is layered - protocols, encapsulation, OSI and TCP/IP; transport and network deliver the bytes - TCP/UDP, IP, routing, NAT; and the link layer crosses each hop - MAC, ARP, Ethernet, Wi-Fi.
One practical security topic closes the round-up: Passwords. A password’s strength is its entropy in bits - \log_2(\text{alphabet}^{\text{length}}) - and length adds entropy faster than exotic symbols: A long passphrase beats a short “complex” one. Four random words from a 2000-word list give \log_2(2000^4) \approx 44 bits - far stronger than P@ss1. Never reuse passwords (one leak then unlocks every other account); in practice, use a password manager for long, unique passwords, and turn on multi-factor authentication wherever it is offered.
Before Lecture 12 (Exam preparation). Go back to any topic that felt shaky in this chapter’s quick checks. Try re-doing a few worksheet questions under time pressure. And come with questions: The final session covers the exam’s format, tools and strategy - it is delivered live and has no chapter in this script.
11.7 Closing words
That is the course. Eleven chapters ago it opened with a claim that may then have sounded like a slogan: Everything is bits, and only the encoding differs. By now you have seen the claim carry a number system, a text standard, a processor, an operating system and the entire internet - and you have traced one click from a keypress, through gates, cycles, system calls, sockets, datagrams and frames, to a rendered page. Walk into the exam with that trace in your head, answer everything, and trust the breadth of what you have built this semester. Good luck - and remember the monster stayed small because you fed it nothing.