Dense, scannable lookup pages for commands, codes and syntax.
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.
Transfer time, uptime nines and downtime, series and redundant availability, RAID capacity and IPv6 subnet sizes with worked examples and the calculators.
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.
Octal and symbolic chmod modes, what each digit means, how to read ls -l, special bits and umask, on one page.
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.
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.
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.
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.
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.
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.