Unlock Computer Networking Fundamentals: From Circuit Switching to OSPF and VLANs

Network Basics

Circuit Switching : A dedicated communication channel is established before data transmission and remains until the session ends.

Packet Switching (also called message or datagram switching): Data is divided into packets, each routed independently through the network.

Network Protocol : A set of standards or rules defining how data is transmitted over a network.

Protocol Stack : Concrete implementations of network protocols.

World Wide Web (WWW) : Web resources accessed via URLs over HTTP/HTTPS on the Internet.

LAN (Local Area Network) : A network connecting devices within a limited area.

MAN (Metropolitan Area Network) : A larger network covering a city or large campus.

WAN (Wide Area Network) : A network spanning large geographic regions.

Internet : The global network of interconnected devices using various Internet protocols.

IoT (Internet of Things) : Devices with embedded chips connected to a network for data exchange.

Cloud Computing : Providing shared processing resources over the Internet.

Big Data : Analyzing massive data sets to derive accurate predictions.

SDN (Software‑Defined Networking) : Separates control and data planes, enhancing programmability.

Data Plane / Forwarding Plane : Handles packet forwarding decisions in network devices.

Control Plane : Manages control functions for forwarding.

Operating Systems

Operating System : Software platform installed on devices that abstracts hardware differences and provides portability for programs.

GUI (Graphical User Interface) : Allows users to interact with devices via visual icons and windows.

CLI (Command‑Line Interface) : Users issue textual commands to operate devices.

RAM : Volatile memory used for temporary storage; data is lost when power is removed.

Flash : Non‑volatile storage similar to a hard drive, used to store the OS and other files.

NVRAM : Non‑volatile RAM that retains startup configuration after power loss.

Console Interface : Direct serial connection to a device for local management.

Network Protocols

OSI Model

The OSI model divides communication functions into seven logical layers.

TCP/IP Model

The TCP/IP (Internet protocol stack) consists of four layers.

Key Layers

Application Layer : OSI layer 7 / TCP/IP layer 4; closest to the user, handling application protocols.

Transport Layer : OSI layer 4 / TCP/IP layer 3; governs data transfer processes.

Network Layer : OSI layer 3; determines routing of packets between source and destination.

Data Link Layer : OSI layer 2; defines data transfer between directly connected nodes and handles error detection/correction.

Physical Layer : OSI layer 1; specifies the physical transmission standards.

Network Access Layer

Twisted Pair : Two insulated wires twisted together to reduce interference.

Fiber Optic : Glass fibers transmitting light via total internal reflection.

IEEE 802.3 : Ethernet standard for wired networks.

IEEE 802.11 : Wireless LAN standard.

Parity Check : Verifies the odd/even count of ‘1’ bits to detect errors.

Checksum : Validates data integrity by comparing calculated and received sums.

CRC (Cyclic Redundancy Check) : Polynomial division method for error detection.

Shared Ethernet : All devices share a collision domain and contend for transmission.

Binary : Base‑2 numeral system using only 0 and 1.

Hexadecimal : Base‑16 system using digits 0‑9 and letters A‑F.

Collision Domain : Network segment where only one device can transmit at a time.

Switched Ethernet : Each port forms its own collision domain, eliminating contention.

MAC Address : 48‑bit hardware address displayed in hexadecimal.

Broadcast Domain : Area where broadcast frames are received by all nodes.

Edge Port : Rapid‑convergence port in RSTP.

Shared Port : Half‑duplex port sharing a collision domain.

MSTP : Multiple Spanning Tree Protocol, running multiple instances of STP.

Switching Networks

Collision : Simultaneous transmissions cause interference, preventing proper reception.

Hub : Multi‑port device that repeats incoming signals to all other ports (physical‑layer).

Bridge : Two‑port data‑link device that learns MAC addresses and forwards frames, separating collision domains.

Switch : Multi‑port bridge where each port is an independent collision domain.

Switching Capacity : Maximum data throughput of a switch (bit/s).

Packet Forwarding Rate : Number of packets a switch can forward per second (pps).

Interface Speed : Bit rate each port can handle.

Duplex Modes : Full‑duplex (simultaneous send/receive) and half‑duplex (cannot send and receive at the same time).

VLAN

VLAN : Virtual LAN that logically partitions a physical LAN to reduce broadcast traffic and improve security.

VLAN Tag : 4‑byte identifier inserted into Ethernet frames to indicate VLAN membership.

Tagged Frame : Frame carrying a VLAN tag, typically used between switches.

Untagged Frame : Frame without a VLAN tag, usually from end devices.

PVID : Default VLAN ID assigned to an access port.

Access Port : Connects end devices; frames are sent untagged.

Trunk Port : Connects switches; frames are sent tagged.

Hybrid Port : Can operate as both access and trunk.

Static VLAN : Manually configured VLAN.

Dynamic VLAN : VLAN learned automatically by the switch.

Spanning Tree Protocol (STP)

Root Bridge : Central switch that becomes the root of the spanning tree.

Root Port : Port on a non‑root switch that leads toward the root bridge.

Designated Port : Port chosen to forward traffic on a network segment.

Backup Port : Blocked port that can become a designated port if the root port fails.

BPDU (Bridge Protocol Data Unit) : Messages exchanged by STP to share bridge ID, root ID, and path cost.

Configuration BPDU : Sent by the root bridge periodically (Hello time).

Topology Change Notification (TCN) BPDU : Sent when a change is detected, prompting recalculation.

Bridge ID : Combination of STP priority and MAC address.

Root Path Cost : Sum of port costs from a switch to the root bridge.

Port ID : Combination of port priority and number.

Edge Port Function : Allows immediate transition to forwarding state (fast convergence).

P/A Mechanism : Point‑to‑point port fast‑switching without delay.

Point‑to‑Point Port : Full‑duplex port.

Shared Port : Half‑duplex port.

MSTP : Multiple Spanning Tree Protocol supporting multiple instances.

Network Layer

IPv4 : Widely used Internet protocol version 4.

IPv6 : Next‑generation Internet protocol with a vastly larger address space. 2001:1111:0100:000a:0000:00bc:2500:0a0b Packet Fragmentation : Splitting oversized packets to fit the MTU.

Routing : Process of selecting paths in a routing table to forward packets.

Routing Table : Data structure storing routes used by routers.

Routing Protocol : Defines how routers exchange route information (e.g., RIP, OSPF).

Distance‑Vector Routing : Routers share distance and direction metrics.

Link‑State Routing : Routers share complete topology information.

Longest Prefix Match : Router selects the most specific route when multiple matches exist.

Subnetting

Subnet Mask : 32‑bit pattern indicating the network portion of an IPv4 address.

Network Portion : High‑order bits identifying the network.

Host Portion : Low‑order bits identifying the host.

Classful Addressing : Divides IPv4 into A, B, C, D classes with fixed network lengths.

Classless Addressing : Allows arbitrary network prefix lengths.

Unicast : One‑to‑one communication.

Multicast : One‑to‑many communication using group addresses.

ARP : Resolves an IP address to a MAC address.

Static Routing

Route Entry : Matches destination IP to an outgoing interface and next‑hop.

Route Preference : Determines the best route when multiple exist.

Route Metric : Numerical value used to select the optimal route among equals.

Directly Connected Route : Automatically created for interfaces that are up.

Static Route : Manually configured route.

Default Route : Route with mask 0.0.0.0/0 that matches any destination.

Dynamic Route : Learned via a routing protocol.

Floating Static Route : Backup static route that becomes active upon primary failure.

Summary Route : Aggregates multiple subnets into a single route.

Dynamic Routing

Classful Routing Protocol : Does not include subnet mask in updates.

Classless Routing Protocol : Includes subnet mask in updates.

Link‑State Advertisement (LSA) : Method used by link‑state protocols to share topology.

RIP (Routing Information Protocol) : Distance‑vector protocol.

Update Timer : Periodic interval (default 30 s) for sending RIP updates.

Age Timer : Time after which a route is marked unreachable (default 180 s).

Garbage‑Collection Timer : Time after which an unreachable route is removed (default 120 s).

Split Horizon : Prevents a router from advertising a route back out the interface it was learned on.

Poison Reverse : Advertises a route as unreachable back to the source.

Route Poisoning : Actively advertises a failed route as unreachable.

Triggered Update : Sends an immediate update when a topology change occurs.

OSPF

OSPF Neighbor Table : Records OSPF neighbors and their states.

OSPF Topology Table : Link‑state database containing LSAs from all routers in the area.

Network Types : Broadcast, point‑to‑point, NBMA, point‑to‑multipoint.

Router ID : Unique identifier for a router within an OSPF domain.

DR and BDR : Designated Router and Backup Designated Router for multi‑access networks.

Link‑State Messages (LSAs) : Used to synchronize the link‑state database.

OSPF Backbone Area : Area 0, which all other areas must connect to.

Internal Router : All interfaces belong to the same OSPF area.

Backbone Router : Has an interface in the backbone area.

ABR (Area Border Router) : Connects multiple OSPF areas.

ASBR (Autonomous System Boundary Router) : Injects external routes into OSPF.

OSPF Virtual Link : Connects a non‑backbone area to the backbone when no physical link exists.

Router LSA (Type‑1) : Generated by every router, flooded within its area.

Network LSA (Type‑2) : Generated by the DR for a broadcast network.

Summary LSA (Type‑3) : Generated by ABRs to summarize routes to other areas.

ASBR Summary LSA (Type‑4) : Describes routes to ASBRs.

External LSA (Type‑5) : Advertises routes from outside the OSPF domain.

NSSA External LSA (Type‑7) : Used in Not‑So‑Stubby Areas.

Transport Layer

TCP : Connection‑oriented protocol providing reliable byte‑stream service.

UDP : Connectionless protocol offering best‑effort datagram delivery.

Port Numbers : Range 0 ~ 65535; used to identify application processes.

Well‑Known Ports : 0 ~ 1023; reserved for standard services.

Registered Ports : 1024 ~ 49151; assigned for user applications.

Dynamic/Private Ports : 49152 ~ 65535; chosen by clients for outbound connections.

Socket : Combination of IP address and port number uniquely identifying a communication endpoint.

TCP Connection : Established via a three‑way handshake before data transfer.

Three‑Way Handshake : SYN, SYN‑ACK, ACK exchange to open a TCP session.

Sliding Window : Flow‑control mechanism allowing the receiver to advertise available buffer space.

Application Layer

Client‑Server Model : Servers provide services to client applications.

P2P Model : Peers act both as clients and servers.

Client : Initiates requests for services.

Server : Responds to client requests.

Telnet : Unencrypted remote‑login protocol.

Shell : Command‑line interface for interacting with an operating system.

SSH : Secure remote‑login protocol providing encryption.

DHCP : Dynamically assigns IP configuration to clients.

DNS : Resolves domain names to IP addresses.

Web (HTTP/HTTPS) : Hypertext Transfer Protocol (secure version uses TLS/SSL) for delivering web content.

SMTP : Simple Mail Transfer Protocol for sending email.

POP3 : Retrieves email from a server to a client.

Network Management

Directory : Hierarchical file system structure similar to folders.

Startup Configuration File : Saved device configuration loaded at boot.

VTY (Virtual Terminal) Interface : Provides remote management access via Telnet or SSH.