Cheat Sheets

Every IPv4 prefix from /8 to /32 with its subnet mask, wildcard mask, address count and usable hosts, plus private and special ranges and quick sums.

The port numbers behind the web, mail, SSH, DNS and databases, with TCP or UDP, the three port ranges and which ports to keep off the internet.

DNS record types with their type numbers, purpose and zone-file examples, from A, AAAA, MX and TXT to SRV, CAA, HTTPS and DNSSEC records, plus response codes.

IPv6 special prefixes, from ::1 and fe80::/10 to fc00::/7, ff00::/8 and 2001:db8::/32, with multicast groups, notation rules and EUI-64, checked with Python.

Transfer time, uptime nines and downtime, series and redundant availability, RAID capacity and IPv6 subnet sizes with worked examples and the calculators.

Every standard SMTP reply code with its meaning, how the three digits are built, the enhanced status codes bounces carry and what to do with the common ones.

Every CSP Level 3 directive with what it controls and what it falls back to, the source keywords, meta-tag limits and starter policies.

The HTML entities you actually use, with name, decimal code and meaning, which five characters to escape and how entities are decoded.

Request and response headers, Cache-Control directives, security headers, cookie attributes and CORS headers, each with what it does and typical values.

The HTTP status codes you will actually meet, grouped by class, with the pairs people confuse (401 vs 403, 301 vs 308) and tips for choosing codes in an API.

The MIME types for web pages, data, images, audio, video, fonts and archives, with the rules that cause wrong-type bugs and how to check uploads.

The eight CVSS v3.1 base metrics with their weights, the severity bands, six worked vector scores and how to use a score without misreading it.

Digest sizes of MD5, SHA-1, SHA-2, SHA-3 and BLAKE2, key and block sizes of AES, ChaCha20 and 3DES, password hashes and signature algorithms, with status.

The OpenSSL commands people actually use: hashes and random bytes, keys, certificate requests, reading certificates, converting formats and testing a server. Each one was run against OpenSSL 3.5.

Record syntax for SPF mechanisms and qualifiers, DKIM signature and key tags (RFC 6376) and DMARC tags (RFC 7489 and RFC 9989), with examples.

Octal and symbolic chmod modes, what each digit means, how to read ls -l, special bits and umask, on one page.

The five cron fields, special characters, @daily-style macros and worked schedules with their next run times, plus how Quartz cron differs.

The curl options you actually use: methods, headers, data, JSON, forms, auth, cookies, redirects, downloads, TLS, proxies, timing and debugging, with examples.

ISO 8601 patterns, strftime codes, Unix time units and the leap-year, week-number and daylight-saving traps, every value produced by running code.

The Git commands you actually use, grouped by task, plus a table for undoing the usual mistakes and habits that keep history clean.

The keyboard shortcuts worth learning for editing, browsers, moving through text and the system, with Windows, Linux and macOS side by side.

The terminal commands for files, search, text, processes, disk, network and archives, plus pipes, redirection, shell shortcuts and the commands to be careful with.

Markdown syntax for headings, emphasis, links, images, lists, quotes, code and tables, with tips and the differences between Markdown flavours.

Regex syntax in one page: character classes, anchors, quantifiers, groups, lookaround, flags and ready-to-use patterns, every example tested.

What MAJOR.MINOR.PATCH means, how ^, ~, x-ranges, hyphen ranges and || expand, how prerelease versions sort and which versions each range accepts, checked against npm semver 7.8.5.

SELECT, joins, grouping, INSERT, UPDATE, DELETE, upserts, CREATE TABLE, indexes, transactions and window functions, run in SQLite, plus dialect differences.

Every TOML value type, tables, arrays of tables, dotted keys, strings and dates on one page, with a complete example that was parsed with Python 3.12's tomllib.

Quick reference for YAML: scalars, strings, lists, mappings, multi-line text, anchors, multiple documents and the values that parse differently than you expect.

All 95 printable ASCII characters with decimal and hex codes, the control characters you will meet, and handy patterns like the 32 gap between cases.

0 to 31 in decimal, binary, octal and hex, the powers of two, hand-conversion shortcuts and the Base32 alphabet, with links to every converter.

Every SI prefix from quecto to quetta, the binary prefixes (KiB, MiB, GiB), how far each binary unit drifts from its decimal twin, bits versus bytes, and why a 1 TB drive shows up as 931 GiB.

The full International Morse table for letters, digits and punctuation, the timing rules, SOS and prosigns, matching the site's Morse encoder and decoder.

The 26 ICAO spelling words from Alfa to Zulu with pronunciation, the digit words and tips for spelling things out clearly over a poor line.

The seven Roman symbols, the six subtractive pairs, how to read and write any number from 1 to 3999 and the rules, checked against the site converter.

CSS units, shorthands, shadows, gradients, clamp() fluid type, grid one-liners, easing curves, specificity and contrast, using values from the CSS generators.

All 148 CSS named colors with their hex and RGB values, read back from a real browser, plus the gray/grey aliases and how to convert between formats.

Computed table of aperture and shutter-speed pairs for EV 8 to 16 at ISO 100, a lookup of typical scenes, the ISO shift rule and the 500 and NPF star-trail rules.

ISO 216 A and B series, envelope sizes and US paper sizes in millimetres and inches, why every A size is half the one before and the pixel size of an A4 page at common resolutions.

Screen size and pixel density, print resolution, hyperfocal distance and depth of field, and video file size from bitrate, with worked examples.

Exponent and log rules, the quadratic formula, GCF and LCM, factorials, sequences, determinants and complex numbers, each with a worked example.

Area, perimeter, volume, surface area, Pythagoras, trigonometry, distance and slope formulas, each with a worked, recomputed example.

Percent of, percent change, percent error, percentage points and the reverse percentage, each with a worked example and the mistakes to avoid.

Mean, standard deviation, z-score, confidence intervals, chi-square, regression and probability formulas with one checked worked example for each.

Ohm's law, power, series and parallel resistors, voltage dividers, LED resistors, cable voltage drop, energy cost and battery runtime with worked examples.

Volume, paint, drywall, board feet, roof pitch, stairs, fence, pool and insulation formulas with worked examples that match the calculators.

Typical grams per US cup, tablespoon and teaspoon for 20 common baking ingredients, from the same table the Cups to Grams converter uses, with notes on why volume measuring varies.

Recipe scaling, baker's percentages, flour from total dough weight, pizza area and price per square inch and oven temperature conversion.

Density, weight, moles, molarity, dilution, the ideal gas law, projectile range and half-life, each with a worked example matching the calculators.

Exact conversion factors for length, mass, volume, area, speed, energy, power, pressure and temperature, computed from the legal definitions.

Fuel cost, mpg and L/100 km, tire diameter and speedometer error, horsepower from torque or trap speed and EV charging cost, with worked examples.

The heat index, wind chill and dew point formulas with their valid ranges, plus °C/°F conversion, each recomputed and matching the site calculators.

The interest, loan, investing and everyday-money formulas behind our calculators, each with a worked example you can check against the calculator.

BMI, BMR, TDEE, macros, body surface area, heart-rate targets, one-rep max and blood-pressure formulas, each checked against the matching calculator.

A1C to glucose, blood pressure bands, cholesterol, kidney function, QTc, blood alcohol and IV drip rate with worked examples. Screening only, not medical advice.

Finish times for 5K, 10K, half marathon and marathon at paces from 3:30 to 8:00 per kilometre, with the matching pace per mile, computed with the same code as the calculators.

EXIF tag IDs in hex with name, data type and meaning: camera, date and time, exposure, GPS and orientation values, as used in photo forensics.

Magic bytes of images, documents, archives, media, executables and forensic artefacts: hex signature, offset and extension, common ones checked on real files.

The Windows Security, System and PowerShell event IDs used in investigations, with logon types, 4625 failure codes and the log each one lives in.

Where Windows records autoruns, USB devices, UserAssist, ShellBags, RecentDocs, RunMRU, BAM, ShimCache, Amcache, services and networks: key paths and hives.