Guides to networks, passwords, QR codes, VAT and images
Each guide explains the idea behind one of the tools: what the numbers mean, the rules that catch people out, and what is worth checking. They are checked against the published standards and updated when those change.
Networks
- IP addresses explained: IPv4, IPv6 and the reserved rangesWhy there are two kinds of address, why a website sees only one of yours, and the ranges that never appear on the public internet.
- Subnets and CIDR explained: prefixes, masks and usable hostsWhat the number after the slash means, how many hosts each prefix holds, and how to split one network into several.
- DNS records explained: A, AAAA, CNAME, MX, TXT and NSHow a name becomes an address, what each common record type holds, and why a change can take hours to reach everyone.
- Is a website down, or is it just you? How to check, step by stepThe domain, DNS, the IP address and the web server: where a site can fail, how to tell which one, and why a working site can look down.
- HTTP status codes and Cloudflare errors: what 403, 404, 429, 500, 503 and 521 meanThe three-digit code every web server sends: which mean the site works, which mean it refused you, and which mean it is down.
Security
- Password strength explained: length, entropy and passphrasesWhy a longer password beats one full of symbols, how strength is counted in bits, and what the 2025 NIST rules changed.
- Hash functions explained: SHA-256, checksums and password hashingWhat makes a hash secure, why MD5 and SHA-1 are retired, how to check a download, and why passwords need a slow, salted hash.
- JSON Web Tokens explained: claims, signatures and safe validationWhat a JWT carries, why decoding is not verifying, the checks a server must make, and the attacks that skip them.
For developers
- Unix timestamps and time zones explainedWhat the number behind a date means, how to tell seconds from milliseconds, and why the same instant has a different local time everywhere.
- Counting dates: days between dates, leap years and ISO weeksHow many days lie between two dates, which years are leap years, which week a date falls in, and why the answers are not always what you expect.
- UUIDs explained: v4, v7 and choosing the right versionHow to read a UUID, why version 4 is random and version 7 sorts by time, and which one suits a database key.
- QR codes explained: capacity, error correction and print sizeHow much a QR code holds, what the four error correction levels do, and how to keep a printed code easy to scan.
- Common JSON errors and how to fix themTrailing commas, single quotes, comments and other mistakes that break JSON, plus how to read the parser's error message.
- CSV files explained: delimiters, quotes, encoding and ExcelCSV looks like the simplest format there is, yet files break between programs. The rules, the separator and encoding problems, and what Excel changes.
- Text encodings explained: UTF-8, Base64 and URL encodingUTF-8, Base64 and percent-encoding solve three different problems and are easy to mix up. What each one does, with byte-by-byte examples, and which one you need.
Calculations
PDF & documents
Images
- Image formats explained: JPEG, PNG, WebP, AVIF and HEICWhich format to use for what, why an iPhone photo will not open elsewhere, and how to shrink an image without it looking worse.
- Image size explained: pixels, DPI, file size and qualityPixels, DPI and file size are three different things. What each one changes, and how to pick dimensions and quality with measured numbers instead of guesses.
- Phone photos explained: HEIC, EXIF metadata and your privacyA phone photo carries the time, the device and often the exact place it was taken. How to check what yours reveal and remove it before sharing.