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Tools (25)

Reference (171 entries)

Web & HTTP

HTTP Status Codes — Meanings, Causes and Fixes

HTTP Status Codes — Meanings, Causes and FixesSet overview — 30 entries 100 Continue100 Continue is an interim response that tells the client the request headers were accepted and the body can be sent. 101 Switching Protocols101 Switching Protocols confirms that the server is switching to a protocol requested in the Upgrade header. 200 OK200 OK means the request succeeded and the response body contains the representation the client asked for. 201 Created201 Created means the request succeeded and a new resource was created, usually identified by a Location header. 204 No Content204 No Content means the request succeeded but there is no body to return. 206 Partial Content206 Partial Content returns one requested byte range of a resource instead of the whole representation. 301 Moved Permanently301 Moved Permanently says the resource now lives at a new URL and future requests should use that URL. 302 Found302 Found says the resource is temporarily available at another URL, and the original URL should remain the address clients use. 303 See Other303 tells the client to fetch another URL with GET, usually after a form submission. 304 Not Modified304 Not Modified tells a client that its cached copy is still fresh, so no body needs to be transferred. 307 Temporary Redirect307 Temporary Redirect moves a request to another URL while preserving its method and body. 308 Permanent Redirect308 Permanent Redirect is a permanent move that preserves the request method and body. 400 Bad Request400 Bad Request means the server cannot process the request because the client sent something malformed. 401 Unauthorized401 Unauthorized means the request lacks valid authentication credentials for the target resource. 403 Forbidden403 Forbidden means the server understood the request but refuses to authorize it. 404 Not Found404 Not Found means the server cannot find a current representation for the target URL. 405 Method Not Allowed405 Method Not Allowed means the URL exists but does not support the HTTP method used in the request. 409 Conflict409 Conflict means the request conflicts with the current state of the target resource. 410 Gone410 Gone means the target resource is permanently unavailable and the server has no forwarding address. 412 Precondition FailedA conditional request failed because the resource no longer matches the client’s validator. 413 Content Too LargeThe request body exceeds the limit the server is willing to process. 416 Range Not SatisfiableA Range request asks for bytes outside the resource, or the server cannot satisfy the range. 422 Unprocessable Content422 Unprocessable Content means the request was well formed but its instructions could not be processed. 426 Upgrade RequiredThe server refuses the request until the client switches to a required protocol. 429 Too Many Requests429 Too Many Requests means the client has sent too many requests in a given amount of time. 500 Internal Server Error500 Internal Server Error means the server hit an unexpected condition and could not complete the request. 501 Not ImplementedThe server does not recognize or support the request method. 502 Bad Gateway502 Bad Gateway means a gateway or proxy received an invalid response from the upstream server. 503 Service Unavailable503 Service Unavailable means the server is temporarily unable to handle the request, often due to overload or maintenance. 504 Gateway Timeout504 Gateway Timeout means a gateway or proxy did not receive a timely response from an upstream server.

HTTP Header Reference

HTTP Header ReferenceSet overview — 15 entries HostHost names the site the client wants to reach; it is what makes virtual hosting possible. Content-TypeContent-Type states the media type and optional parameters of the request or response body. Content-LengthContent-Length is the exact body size in bytes, used to delimit messages and detect truncation. Cache-ControlCache-Control tells browsers and shared caches whether, and for how long, they may store a response. ETagETag identifies a specific version of a representation so caches and clients can revalidate it cheaply. AuthorizationAuthorization carries credentials that prove the client may access the resource. WWW-AuthenticateWWW-Authenticate describes the authentication scheme a 401 response expects. CookieCookie sends stored name-value pairs from the client back to the server. Set-CookieSet-Cookie asks the client to store a cookie and defines its scope, lifetime and security attributes. AcceptAccept lists the media types the client prefers for the response. Accept-EncodingAccept-Encoding lists the compression formats the client can decode. User-AgentUser-Agent identifies the client software that made the request. RefererReferer tells the server which page linked to the requested resource. Content-Security-PolicyContent-Security-Policy restricts which sources a page may load and execute. Strict-Transport-SecurityStrict-Transport-Security tells browsers to use HTTPS for this host for a stated period.

MIME Types and Content Types — Reference

Networking

Network Port Reference

Network Port ReferenceSet overview — 21 entries 20 — FTP dataPort 20 carries FTP data in active mode, from server to client. 21 — FTP controlPort 21 carries FTP commands and responses, including credentials in plain FTP. 22 — SSHPort 22 carries SSH: encrypted remote shell, SFTP, SCP and port forwarding. 23 — TelnetPort 23 is the legacy Telnet service, which sends everything including passwords in clear text. 25 — SMTPPort 25 is the classic SMTP port for server-to-server mail delivery. 53 — DNSPort 53 resolves domain names, over UDP for queries and TCP for large answers or zone transfers. 80 — HTTPPort 80 is the default port for plain HTTP. 110 — POP3Port 110 is the POP3 mailbox protocol, traditionally download-and-delete. 143 — IMAPPort 143 is IMAP, which keeps mail on the server and synchronises folders across devices. 443 — HTTPSPort 443 is the default port for HTTPS and for HTTP/3 over QUIC. 445 — SMBPort 445 carries SMB file sharing on Windows networks and must never face the internet. 587 — SMTP submissionPort 587 is the authenticated submission port for sending mail from a client. 993 — IMAPSPort 993 is IMAP over TLS, encrypted from the first byte. 995 — POP3SPort 995 is POP3 over TLS, encrypted from the first byte. 3306 — MySQLPort 3306 is the default MySQL and MariaDB listener. 3389 — RDPPort 3389 is Microsoft Remote Desktop, a frequent target for brute force and ransomware. 5432 — PostgreSQLPort 5432 is the default PostgreSQL listener. 6379 — RedisPort 6379 is the default Redis listener, and an unauthenticated Redis is a common breach. 8080 — HTTP alternatePort 8080 is a common alternate HTTP port for proxies, development servers and admin panels. 8443 — HTTPS alternatePort 8443 is a common alternate HTTPS port for admin interfaces and application servers. 27017 — MongoDBPort 27017 is the default MongoDB listener and a repeated target of mass ransom campaigns.

Encoding & data

QR Code Reference

QR Code ReferenceSet overview — 13 entries QR error correctionFour levels — L, M, Q and H — trade payload capacity for tolerance of dirt, damage and glare. QR code versionVersions 1 to 40 set the grid size, from 21×21 to 177×177 modules. QR code capacityMaximum payload by mode and error-correction level, and why UTF-8 text uses more space than digits. QR encoding modesNumeric, alphanumeric, byte, kanji and ECI decide how many bits each character costs. QR quiet zoneFour empty modules on every side, required for the scanner to find the symbol. QR mask patternOne of eight XOR patterns applied to data modules to avoid large blank or blocky areas. QR format informationFifteen bits that tell the scanner the error-correction level and mask, protected by BCH. QR structured appendSplitting one payload across up to sixteen linked symbols. Micro QR codeA compact QR variant with one finder pattern, for very short payloads. Wi-Fi QR code formatThe WIFI: payload string that joins a network without typing the password. QR module sizeHow large each square must be printed for a camera to resolve it. QR code print sizeChoosing a physical size from module count, scan distance and surface. QR codes for binary and file dataWhen a QR code carries bytes rather than text, and why files rarely belong in one.

UUID Reference

UUID ReferenceSet overview — 11 entries UUID format128 bits written as 32 hex digits in five hyphenated groups. UUID v4122 random bits — the default choice for identifiers that need no ordering. UUID v7A 48-bit Unix millisecond timestamp followed by random bits, for sortable identifiers. UUID v1Timestamp plus clock sequence plus node, historically a MAC address. UUID v3 and v5Deterministic identifiers derived from a namespace and a name via MD5 or SHA-1. Nil UUID00000000-0000-0000-0000-000000000000, the all-zero UUID. Max UUIDffffffff-ffff-ffff-ffff-ffffffffffff, the all-one UUID. UUID version and variant bitsThe fixed bits that tell a parser how the identifier was built. UUID collision riskWhy 122 random bits is safe at scale, and when name-based IDs collide deliberately. UUID vs ULIDTwo 128-bit identifiers with different text encodings and ordering guarantees. GUID vs UUIDMicrosoft's GUID is a UUID with platform-specific formatting and byte order.

Design & frontend

Color Format Reference

Color Format ReferenceSet overview — 10 entries Hex color#RRGGBB notation, its shorthand forms and the 8-digit alpha variant. RGB colorThree sRGB channels from 0 to 255, plus optional alpha. HSL colorHue in degrees, saturation and lightness in percent. HSV / HSB colorHue, saturation and value — the model behind most color pickers. CMYK colorSubtractive cyan, magenta, yellow and key (black) for print. CSS named colorsAbout 148 keyword colors, from aliceblue to yellowgreen. Alpha channelOpacity from fully transparent to fully opaque, in 0–1 or 0–255. WCAG contrast ratioThe 4.5:1 and 3:1 thresholds for readable text and UI components. Relative luminanceThe WCAG brightness measure behind contrast ratios. sRGB gamutThe default web color space, and what happens outside it.

CSS Unit Reference

CSS Unit ReferenceSet overview — 10 entries pxThe CSS pixel is a reference unit: 96 CSS pixels equal one inch. remOne rem equals the root element's font size, typically 16px. emOne em equals the current element's computed font size and compounds through nesting. ch and exch is the width of the digit zero; ex is the font's x-height. vw and vhOne vw is 1% of the viewport width; one vh is 1% of the viewport height. dvh, svh and lvhDynamic, small and large viewport units describe the height as browser chrome shows or hides. vmin and vmaxvmin is 1% of the smaller viewport side; vmax is 1% of the larger side. %A percentage resolves against a property-specific reference, often the parent's size. frThe fr unit distributes free space in a CSS grid track. calc, min, max and clampThese functions combine lengths and bound them without JavaScript.

Time & scheduling

Cron Expression Reference

Cron Expression ReferenceSet overview — 10 entries Cron field orderA five-field cron expression runs minute, hour, day-of-month, month and day-of-week, in that order. Cron step valuesA slash adds a step to a range: */5 means every fifth value starting at the range's lower bound. Cron ranges and listsCommas combine values and hyphens define inclusive ranges; both can be mixed and both may use month or weekday names. Month and weekday namesCron accepts JAN–DEC and SUN–SAT, case-insensitively, but the numbering of Sunday differs between dialects. Day-of-month and day-of-week togetherWhen both day fields are restricted, Vixie-style cron runs the job if either field matches. Cron special stringsMacros such as @daily and @reboot replace common five-field schedules in Vixie-style cron. Cron and time zonesCron schedules run in the system's local time zone unless the implementation offers CRON_TZ or TZ support. Cron vs systemd timersCron is one line per job; systemd timers separate a unit from its schedule and can catch up on missed runs. Six-field cron dialectsQuartz, Spring, AWS EventBridge and Jenkins extend cron with seconds or other fields, and they do not agree on order. Common cron schedulesThe expressions people ask for most: every five minutes, hourly, daily, weekdays at 09:00 and first of the month.

Text & patterns

Regular Expression Reference

Regular Expression ReferenceSet overview — 10 entries Character classesSquare brackets match one character from a set, with ranges, negation and shorthand classes such as \d and \w. QuantifiersQuantifiers repeat the previous atom: * zero or more, + one or more, ? optional, and {n,m} an explicit count. Greedy and lazy matchingGreedy quantifiers take as much as possible; adding ? makes them lazy and take as little as possible. Anchors and word boundariesAnchors match positions, not characters: ^ and $ for line or string edges, \b for a word boundary. Capturing groupsParentheses group a subpattern and capture its text; groups are numbered by the order of their opening parenthesis. Non-capturing groups(?:...) groups a subpattern for quantifiers and alternation without creating a capture. Lookahead and lookbehindLookaround asserts what comes before or after a position without consuming characters. BackreferencesA backreference matches the same text captured earlier by a group, such as \1 or \k<name>. Regex flagsFlags change how a pattern is interpreted: global, case-insensitive, multiline, dot-all, Unicode, sticky and indices. Catastrophic backtrackingNested quantifiers can make a regex take exponential time, a failure mode known as ReDoS.

Markdown Reference

Markdown ReferenceSet overview — 10 entries HeadingsATX headings use one to six # characters; setext headings underline text with = or -. Paragraphs and line breaksA blank line starts a new paragraph; a single newline is usually just a space. EmphasisAsterisks and underscores mark italic and bold text, with rules about word boundaries. LinksInline, reference and automatic links connect text to a URL. ImagesAn image is a link with a leading ! and the alt text in square brackets. ListsOrdered and unordered lists use markers at the start of a line, with indentation for nesting. CodeBackticks mark inline code; triple backticks create fenced blocks with optional language info. BlockquotesA > marker at the start of a line creates a blockquote; markers nest. TablesGFM tables use pipes and a delimiter row to define columns and alignment. Task listsGFM task lists add checkboxes to list items with [ ] and [x].

Security

Hash Algorithm Reference

Hash Algorithm ReferenceSet overview — 8 entries MD5MD5 produces a 128-bit digest and is broken for security use. SHA-1SHA-1 produces a 160-bit digest and is deprecated because collisions are practical. SHA-256SHA-256 produces a 256-bit digest and is the default choice for integrity and signatures. SHA-512SHA-512 produces a 512-bit digest and is efficient on 64-bit processors. SHA-3SHA-3 is the Keccak sponge family, standardized separately from SHA-2. bcryptbcrypt is an adaptive password hash based on Blowfish with a tunable cost factor. scryptscrypt is a memory-hard key derivation function with tunable memory and CPU cost. Argon2Argon2 is the Password Hashing Competition winner and the current recommended password hash.

Tools (25)

Generators (4)

Text tools (4)

Developer tools (1)

Converters & encoders (6)

Time & date (2)

Calculators (3)

Reference lookup (3)

Web & network (1)

Security (1)

Reference (171 entries)

Web & HTTP

HTTP Status Codes — Meanings, Causes and Fixes

HTTP Header Reference

MIME Types and Content Types — Reference

Networking

Network Port Reference

Encoding & data

QR Code Reference

UUID Reference

Design & frontend

Color Format Reference

CSS Unit Reference

Time & scheduling

Cron Expression Reference

Text & patterns

Regular Expression Reference

Markdown Reference

Security

Hash Algorithm Reference