Tech+ cheat sheet

1 byte = 8 bits; a nibble = 4 bits; two hexadecimal digits = 1 byte.

Storage prefixes: 1 KB = 1,000 bytes (decimal SI) but 1 KiB = 1,024 bytes (binary); 1 MB = 1,000 KB, 1 GB = 1,000 MB, 1 TB = 1,000 GB in SI terms.

Number systems: binary base 2 (0-1), octal base 8 (0-7), decimal base 10 (0-9), hexadecimal base 16 (0-9 plus A-F where A = 10 and F = 15).

ASCII encodes 128 characters in 7 bits (256 in the extended 8-bit set); Unicode/UTF-8 covers all written languages using 1 to 4 bytes per character.

Bandwidth is maximum capacity in bits per second; throughput is the actual delivered rate; latency is the round-trip delay measured in milliseconds.

Data is raw, unprocessed facts; information is data that has been processed and given context so it can support decisions.

Cloud characteristics: on-demand self-service, broad network access, resource pooling, rapid elasticity, and measured (metered) service.

Vertical scaling adds CPU, RAM, or storage to one machine; horizontal scaling adds more machines; elasticity automatically adjusts capacity to match demand.

Type 1 hypervisors run directly on bare-metal hardware; Type 2 hypervisors run on top of a host operating system; each virtual machine contains a full guest OS.

CPU performance depends on clock speed (GHz), core count, cache size (L1/L2/L3), and instruction set; more cores help parallel workloads, higher clock speed helps single-threaded work.

RAM is volatile and loses its contents on power loss; SSD and HDD storage is non-volatile; DDR generations are keyed differently and are never interchangeable.

RAID 0 = striping with no redundancy; RAID 1 = mirroring with 50% usable capacity; RAID 5 = striping plus distributed parity, minimum 3 disks, survives 1 failure; RAID 10 = mirror plus stripe, minimum 4 disks.

A switch forwards frames by MAC address at Layer 2; a router forwards packets by IP address at Layer 3; a wireless access point bridges Wi-Fi clients onto the wired LAN.

Well-known ports: 22 SSH, 25 SMTP, 53 DNS, 80 HTTP, 110 POP3, 143 IMAP, 443 HTTPS, 3389 RDP.

IPv4 is 32-bit dotted decimal; IPv6 is 128-bit hexadecimal; private ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16; 169.254.0.0/16 is APIPA, assigned when DHCP fails.

Cloud service models: IaaS supplies virtualized compute, storage, and networking; PaaS supplies a managed runtime and development platform; SaaS supplies a finished application over the network.

DNS resolves host names to IP addresses; DHCP automatically assigns IP configuration; NAT translates private addresses to a public address for internet access.

Wi-Fi 6 (802.11ax) operates on both 2.4 GHz and 5 GHz; 2.4 GHz travels farther and penetrates walls better, while 5 GHz provides more channels and higher throughput at shorter range.

Operating system responsibilities: process scheduling, memory management, file system management, device driver control, and the user interface.

File types by purpose: .docx word processing, .xlsx spreadsheet, .pptx presentation, .pdf fixed-layout document, .txt plain text, .csv comma-separated values, .zip compressed archive.

Installation types: clean install wipes existing data, upgrade preserves files and settings, patch fixes a specific defect, rollback reverts to the prior version.

Licensing models: perpetual (one-time purchase), subscription (recurring fee), open source (source code available under a license), proprietary (use restricted by the EULA).

Application categories: productivity, business and finance, collaboration, communication, utility, and specialized tools such as CAD or graphic design.

Containers share the host OS kernel and are sized in megabytes; virtual machines each include a guest OS and are sized in gigabytes.

Browser troubleshooting order: clear cache and cookies, disable extensions, verify system date and time for TLS, then check proxy and DNS settings.

Compatibility depends on OS version, CPU architecture (32-bit vs 64-bit), RAM, and free storage; 64-bit applications cannot run on a 32-bit operating system.

SDLC phases in order: planning, analysis, design, implementation, testing, deployment, maintenance.

Waterfall is sequential with fixed requirements; Agile is iterative with short increments; Scrum uses timeboxed sprints and defined roles; Kanban limits work in progress for continuous flow.

Structured programming constructs: sequence, selection (if/else, switch), and iteration (for, while, do-while).

Data types: integer (whole numbers), float (decimals), string (text), Boolean (true/false), char (single character).

Compiled languages translate all source code to machine code before execution; interpreted languages translate and execute statements line by line at runtime.

Testing levels: unit (single component), integration (combined components), system (whole application), acceptance (user or business validation); regression re-tests existing features after a change.

Flowchart symbols: oval = start/end, rectangle = process, diamond = decision, parallelogram = input/output, arrow = control flow.

REST APIs use HTTP methods GET, POST, PUT or PATCH, and DELETE, and exchange JSON or XML payloads; SOAP is a stricter XML-only protocol.

Version control operations: commit saves a snapshot, branch isolates work, merge combines changes, and a pull request asks for peer review before merging.

A primary key uniquely identifies each row and cannot be null or duplicated; a foreign key references another table's primary key and enforces referential integrity.

Core SQL statements: SELECT reads rows, INSERT creates rows, UPDATE modifies rows, DELETE removes rows; DDL uses CREATE, ALTER, and DROP.

Normalization: 1NF removes repeating groups so values are atomic; 2NF removes partial dependencies on part of a composite key; 3NF removes transitive dependencies between non-key columns.

ACID: atomicity (all or nothing), consistency (valid state transitions only), isolation (concurrent transactions do not interfere), durability (committed data survives failure).

Backup types: full copies all data; differential copies everything changed since the last full backup; incremental copies everything changed since the last backup of any type.

NoSQL categories: document (JSON-like), key-value, column-family, and graph; they are typically chosen for horizontal scale and flexible schemas.

ETL extracts data from sources, transforms it, and loads it into a data warehouse; a data lake instead stores raw data in its native format.

Common SQL data types: INT, DECIMAL, VARCHAR, CHAR, DATE, DATETIME, BOOLEAN, BLOB.

Validation checks that data meets rules such as format or range; verification confirms the data was entered or transferred accurately via double entry or checksums.

CIA triad: confidentiality is protected by encryption and access control, integrity by hashing and checksums, availability by redundancy and backups.

Authentication factors: something you know (password), something you have (token, phone), something you are (biometrics); MFA requires two or more different factor types.

Symmetric encryption such as AES uses one shared key and is fast; asymmetric encryption such as RSA or ECC uses a public/private key pair and solves key distribution.

Hashing (SHA-256) is one-way and verifies integrity; encryption is reversible with the key; encoding such as Base64 provides no security.

Malware types: a virus needs a host file and user action, a worm self-propagates across networks, a trojan disguises itself as legitimate software, ransomware encrypts data for extortion, spyware collects activity.

A firewall filters traffic by rule; an IDS detects and alerts; an IPS detects and blocks; antivirus matches known signatures plus heuristic behavior.

Least privilege grants only the rights needed for a task; zero trust requires verification of every request regardless of network location.

Social engineering variants: phishing (email), smishing (SMS), vishing (voice call), tailgating (physical follow-in), shoulder surfing (watching a screen).

Physical controls include badges, biometric readers, mantraps, locks, and cameras; administrative controls include policies, training, and background checks.