Reference tools on it.you are the lookup tables and quick-reference guides you reach for when you can't remember the exact syntax of a Linux find command, need to know what HTTP status code 422 means, want to check which port MySQL listens on, or need to convert a hex colour to HSL. Rather than searching Google and landing on a low-quality article, these reference pages give you the information directly — formatted for scanning, not reading.

Reference content is kept up to date with current standards. The HTTP status code list covers RFC 9110 (HTTP Semantics), the port reference covers IANA well-known ports (0–1023) and registered ports (1024–49151), and the Linux commands reference covers GNU coreutils with common flag combinations. All pages load instantly and work offline once cached.

Who uses these tools?

Developers

Look up HTTP status codes during API development, find the right HTTP method for an operation, convert colour formats for CSS.

Sysadmins

Reference Linux command flags without man pages, check port assignments for firewall rules, look up MIME types for server config.

IT support teams

Quickly identify HTTP error codes in logs, reference common ports during network troubleshooting.

Students

Study Linux commands, HTTP semantics, and networking fundamentals with structured reference tables.

What's in this category?

Linux Commands Reference

Searchable reference for common Linux commands with exa

Color Format Converter

Convert between HEX, RGB, HSL, and CSS color formats.

Common Ports Reference

Searchable list of all well-known and common network po

HTTP Status Code Reference

Complete reference for all HTTP status codes with expla

Tool types in this category

Type	Count	How it works	Privacy
Browser-only	4	Runs entirely in your browser — no server requests	No data leaves your device

Key concepts

HTTP status codes — Three-digit response codes that indicate the outcome of an HTTP request. 1xx = informational, 2xx = success, 3xx = redirection, 4xx = client error, 5xx = server error. Defined in RFC 9110.

Well-known ports (0–1023) — Port numbers assigned by IANA for specific services. Require root/admin privileges to bind on Unix systems. Examples: 22 (SSH), 53 (DNS), 80 (HTTP), 443 (HTTPS).

MIME types — Multipurpose Internet Mail Extensions types identify file formats in HTTP. Format: type/subtype. Examples: text/html, application/json, image/webp, application/octet-stream.

ASCII — American Standard Code for Information Interchange — a 7-bit character encoding for 128 characters (0–127). The foundation of UTF-8. Printable characters start at 32 (space) and end at 126 (~).

Color models — Different mathematical models for representing colour: RGB (red, green, blue — used in screens), HSL (hue, saturation, lightness — easier for humans to reason about), and HEX (compact RGB representation used in CSS).

Frequently asked questions

What is the difference between HTTP 401 and 403?

401 Unauthorized means the request requires authentication — the client has not identified itself. 403 Forbidden means the server understood the request and knows who the client is, but refuses to authorise it. In practice: 401 = not logged in, 403 = logged in but not allowed.

What is the difference between HTTP 301 and 302?

301 Moved Permanently tells clients and search engines the resource has permanently moved — search engines transfer link equity to the new URL. 302 Found is a temporary redirect — search engines keep indexing the original URL.

What ports are commonly blocked by ISPs?

Port 25 (SMTP) is commonly blocked by residential ISPs to prevent spam. Ports 137-139 (NetBIOS) and 445 (SMB) are frequently blocked for security. Some ISPs also block ports 80 and 443 on residential connections to prevent running servers.

What is the chmod octal notation?

Each digit represents permissions for owner, group, and others. Values: 4=read, 2=write, 1=execute. Add them for combinations: 7(rwx), 6(rw-), 5(r-x), 4(r--). Common patterns: 755 (executable scripts), 644 (regular files), 600 (private keys).

What is a MIME type and where is it used?

MIME types are used in the HTTP Content-Type header to tell the browser how to handle a response body. Misconfigured MIME types cause browsers to render HTML as plain text, block scripts, or mishandle downloads. Always set Content-Type explicitly.

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