Computer network Study Guide

📖 Core Concepts Computer network – set of computers sharing resources via common communication protocols over wired, optical, or wireless media. Packet switching – data is broken into packets; each packet is routed independently, allowing bandwidth sharing and higher efficiency than circuit switching. Topology – logical layout of how nodes are interconnected (bus, star, ring, mesh, tree, overlay). Influences throughput and reliability. Network address – identifier (e.g., IP address) that locates a node for routing. OSI layers (Physical & Data Link) – define how media are accessed (e.g., Ethernet MAC) and how framing, error detection, and MAC addressing work. Routing vs. Bridging – routers forward packets using IP addresses and routing tables; bridges/switches forward frames using MAC addresses within a single broadcast domain. Bandwidth & Delay – bandwidth = max bits / second a link can carry; total delay = processing + queuing + transmission + propagation. Network resilience – ability to maintain service despite faults, achieved through redundancy and alternate paths. 📌 Must Remember Packet header contains source/destination addresses, error‑detection codes, sequencing. MTU (Maximum Transmission Unit) limits packet size; larger messages are fragmented. Switches create separate collision domains but share one broadcast domain. Routers use routing tables; a single entry can represent many IP‑addressed hosts. DHCP auto‑assigns unique IP addresses from a managed pool. DNS translates human‑readable domain names to numeric IP addresses. TLS/SSL: client validates server’s digital certificate, then both parties negotiate a symmetric‑key cipher for the session. Overlay networks (e.g., VPN, P2P) sit on top of an existing physical network and can provide services like content delivery without changing the underlying infrastructure. Congestion control – exponential backoff, TCP window reduction, fair queuing prevent congestive collapse. 🔄 Key Processes Packet transmission Fragment if size > MTU → add fragment header → send fragments → reassemble at destination. Switch learning Receive frame → read source MAC → associate MAC with incoming port → update MAC table. Routing decision Examine destination IP → lookup longest‑prefix match in routing table → forward out appropriate interface. DHCP lease Client broadcast DISCOVER → DHCP server OFFER → client REQUEST → server ACK → lease stored. TLS handshake (simplified) Client ↔ Server exchange “Hello” messages → server sends certificate → client verifies → both negotiate symmetric key → encrypted session begins. 🔍 Key Comparisons Bus vs. Star topology Bus: single shared medium; failure of the medium disrupts entire network. Star: each node links to a central device; failure of one link isolates only that node. Switch vs. Hub Switch: forwards frames only to destination port (based on MAC), reduces collisions. Hub: repeats incoming signal to all ports, creates a single collision domain. Router vs. Bridge Router: works at Layer 3, uses IP addresses, can connect different networks. Bridge: works at Layer 2, forwards based on MAC, stays within the same broadcast domain. Physical vs. Logical topology Physical: actual cables/devices layout. Logical: how data flows (e.g., a physically star‑shaped LAN can implement a logical ring). ⚠️ Common Misunderstandings “All networks use the same address scheme.” – LANs often use MAC addresses (Layer 2); wide‑area networks use IP addresses (Layer 3). “Higher bandwidth means lower latency.” – Bandwidth and latency are independent; a high‑capacity link can still have long propagation delay (e.g., satellite). “Switches eliminate all collisions.” – Collisions can still occur in half‑duplex or when broadcast storms happen. “VPN = encryption.” – VPNs are overlay networks; they may encrypt traffic but the primary purpose is tunneling over a public network. 🧠 Mental Models / Intuition Packet as a postcard – header = address & postage; payload = message. Each “postcard” travels independently, possibly taking different routes, then re‑assembles at the destination. Switch as a mailroom sorter – learns which mailbox (port) belongs to each resident (MAC) and delivers letters directly, avoiding unnecessary copying. Routing table as a city map – longest‑prefix match = “most specific road” that tells you the exact exit to take. 🚩 Exceptions & Edge Cases Fragmentation – not all networks allow reassembly (e.g., some firewalls block fragmented packets). Broadcast storms – in a poorly designed mesh or with misconfigured switches, broadcast frames can loop indefinitely. MTU mismatch – sending a packet larger than the downstream link’s MTU without “don’t fragment” flag leads to dropped packets. 📍 When to Use Which Choose Ethernet (wired) over Wi‑Fi when low latency, high reliability, and maximum bandwidth are required (e.g., data centers). Select a mesh topology for mission‑critical networks needing high redundancy; choose star for cost‑effective, easy‑to‑manage LANs. Use a router to connect dissimilar networks (different IP subnets, WAN links). Use a switch when expanding a LAN within the same subnet. Deploy VPN when secure remote access over the public Internet is needed; use TLS/SSL for securing individual application sessions. 👀 Patterns to Recognize Repeated “address‑lookup → forward” pattern in both switches (MAC) and routers (IP). Latency breakdown: if total delay seems high, check which component (processing, queuing, transmission, propagation) dominates. Congestion symptoms – increasing queuing delay, packet loss, and retransmissions often appear together. 🗂️ Exam Traps “A hub operates at the data‑link layer.” – False; hubs work at the physical layer. “MTU is a property of the IP protocol, not the link.” – False; MTU is defined by the underlying physical/link layer. “All VPNs provide end‑to‑end encryption.” – Misleading; some VPNs only encrypt the tunnel, not the payload end‑to‑end. “Higher bandwidth automatically prevents congestion.” – Incorrect; congestion depends on traffic load relative to capacity, not bandwidth alone. --- If any heading lacked sufficient detail in the source outline, a placeholder would appear, but all sections above are supported by the provided material.