Internet Study Guide

📖 Core Concepts Internet – Global “network of networks” using the TCP/IP suite to move data between devices. TCP/IP Suite – Four layers: Application (e.g., HTTP), Transport (TCP reliable, UDP unreliable), Internet (IP addressing/routing), Link (Ethernet, Wi‑Fi, DSL). ICANN – Coordinates the global IP address space and Domain Name System (DNS). IETF – Writes and publishes protocol standards as RFCs (e.g., IPv4, IPv6, BGP). Tier‑1 ISP – Large telcos that peer with each other; no upstream transit needed. IP Addressing – IPv4 = 32‑bit (≈4.3 B addresses); IPv6 = 128‑bit (practically unlimited). Subnetting / CIDR – Divides a network into smaller logical pieces; notation address/prefix. Routing – Routers forward packets using routing tables; BGP exchanges routes between ISPs; default gateway sends traffic when no specific route matches. Key Services – WWW (hypertext documents), email (asynchronous text), VoIP (voice over IP), CDNs (content close to users). 📌 Must Remember IPv4 exhaustion occurred in 2011 → transition to IPv6. CIDR example: 198.51.100.0/24 → 24‑bit network prefix, 8‑bit host portion. Tier‑1 networks exchange traffic directly; lower tiers buy transit or peer. ICANN manages both IP address allocation and DNS root zones. IETF standards are published as RFCs. BGP = Border Gateway Protocol, the global routing protocol for ISPs. Video bandwidth needs: 480p ≈ 1 Mbit/s, 720p ≈ 2.5 Mbit/s, 1080p ≈ 4.5 Mbit/s. Cyber‑crime cost: $6 trillion (2021) → projected $10.5 trillion/yr by 2025. 🔄 Key Processes Packet Delivery (IP Layer) Source host → source IP → passes to router → router looks up longest‑prefix match in routing table → forwards to next hop → repeat until destination network reached → final router uses ARP/ND to deliver to destination host. Subnet Calculation Given 192.0.2.0/28 → network bits = 28 → host bits = 4 → usable hosts = $2^{4}-2 = 14$ (exclude network & broadcast). BGP Route Advertisement ISP A advertises its prefixes to ISP B → B adds them to its BGP table → chooses best path (based on AS‑PATH length, local pref, etc.) → propagates to peers. Domain Name Resolution (DNS) User types www.example.com → resolver queries root → TLD server → authoritative server → returns IP → browser initiates TCP connection. 🔍 Key Comparisons IPv4 vs IPv6 32‑bit vs 128‑bit address space. IPv4 exhaustion vs abundant IPv6 addresses. IPv4 uses dot‑decimal (a.b.c.d); IPv6 uses hexadecimal colon (2001:db8::1). IPv6 not directly interoperable → requires translation or dual‑stack. Tier‑1 vs Tier‑2 ISP Tier‑1: no upstream transit, peers with other Tier‑1s. Tier‑2: purchases transit, may also peer locally. TCP vs UDP TCP: connection‑oriented, reliable, flow control, congestion control. UDP: connection‑less, no guarantees, low latency (used for streaming, DNS). ⚠️ Common Misunderstandings “Internet = Web” – The Web is just one service (HTTP/HTML) that runs on the Internet. IPv6 automatically fixes all address problems – Transition requires dual‑stack or translation; many networks still run IPv4. All ISPs peer with every other ISP – Only Tier‑1 networks have full‑mesh peering; others rely on transit. BGP is a security protocol – BGP selects routes; it does not authenticate them (hence hijacking risks). 🧠 Mental Models / Intuition Layered cake – Think of the TCP/IP suite as a cake: each layer builds on the one below (Link → Internet → Transport → Application). Address as street address – IP = house number (host) + street (network prefix). Subnetting is carving a larger block into smaller streets. Routing as GPS – Routers are GPS devices that consult a map (routing table) to choose the best road (next hop). 🚩 Exceptions & Edge Cases IPv4‑to‑IPv6 translation – NAT64/DNS64 can allow IPv6‑only clients to reach IPv4 servers. Default gateway – If a host’s routing table contains a more specific route, the default gateway is not used. CIDR with non‑byte‑aligned prefixes – e.g., /13 or /27; mask calculation must follow binary boundaries, not just decimal octets. 📍 When to Use Which Choose IPv6 when deploying new infrastructure that requires a large address pool or wants to avoid NAT. Use TCP for applications needing guaranteed delivery (web pages, file transfer). Use UDP for latency‑sensitive traffic where occasional loss is acceptable (live video, DNS queries). Peer directly (Tier‑1) only if you control a network of global scale; otherwise purchase transit from a Tier‑2/3 ISP. 👀 Patterns to Recognize “/xx” notation → network vs host bits – Larger prefix (e.g., /24) → smaller host pool, typical for LANs. BGP announcements often include “AS‑PATH” – Long AS‑PATH → less preferred route. Video streaming questions often give bandwidth – Map 480p/720p/1080p to 1/2.5/4.5 Mbit/s respectively. Security‑related items: “malware”, “ransomware”, “botnet” → always tied to cyber‑crime cost figures. 🗂️ Exam Traps “The Internet is owned by a single entity.” – False; it is decentralized with no central governing body. “IPv6 eliminates the need for NAT.” – Mostly true, but transition mechanisms may still involve NAT‑like behavior. “All Tier‑1 ISPs have the same speed.” – Speed varies; the key is peering without paying transit, not a fixed rate. “BGP guarantees secure routing.” – BGP lacks built‑in authentication; route hijacking is possible. “The World Wide Web and the Internet are interchangeable terms.” – Incorrect; the Web is a service on the Internet.