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HTTP Fundamentals

Core HTTP primitives and baseline concepts every web engineer should know.

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HTTP Methods
HTTP DELETE Method: Remove Resources
Learn how the HTTP DELETE method works, when to use it, and best practices for deleting resources in REST APIs.
HTTP Methods
HTTP HEAD Method
Learn how HTTP HEAD requests retrieve resource metadata (headers) without downloading the body. Useful for checking existence, size, and modification dates.
HTTP Methods
HTTP OPTIONS Method
Learn how HTTP OPTIONS requests discover server capabilities, supported methods, and handle CORS preflight checks for cross-origin requests.
HTTP Methods
HTTP PATCH Method
Learn how HTTP PATCH requests apply partial modifications to resources. Understand JSON Patch, merge patch formats, and when to use PATCH vs PUT.
HTTP Methods
HTTP PUT Method: Update Resources
Learn how the HTTP PUT method works, when to use PUT vs POST vs PATCH, and best practices for updating resources in REST APIs.
HTTP Methods
HTTP QUERY Method: Safe Requests With a Body (RFC 10008)
HTTP QUERY is a safe, idempotent method with the query in the body. RFC 10008 status, an example exchange, caching rules, and why GET with a body fails.
HTTP Status Codes
100 Continue
The server received the request headers and the client should proceed to send the body. Learn when and how to use 100 Continue for efficient large uploads.
HTTP Status Codes
101 Switching Protocols
The server is switching to a different protocol as requested by the client. Learn about WebSocket upgrades and protocol negotiation.
HTTP Status Codes
102 Processing
The server has accepted the request and is processing it, but no response is available yet. Learn about this WebDAV status code for long-running operations.
HTTP Status Codes
202 Accepted
The request was accepted for processing but not completed yet. Learn when to use 202 for asynchronous operations.
HTTP Status Codes
203 Non-Authoritative Information
203 means a proxy modified the origin response before passing it on. Learn when it is sent, how it differs from 200, caching rules and what clients should do.
HTTP Status Codes
204 No Content
The request succeeded with no response body. Learn when to use 204 No Content for successful operations that don't return data.
HTTP Status Codes
205 Reset Content: What It Does and Browser Support
205 Reset Content tells the client to reset the view that sent the request, like clearing a form. Learn its rules, fetch behaviour and why it is rarely used.
HTTP Status Codes
207 Multi-Status: WebDAV Per-Resource Results
207 Multi-Status carries an XML body with a separate status per resource. See a PROPFIND example, how to parse it, and why the top-level code is not the result.
HTTP Status Codes
300 Multiple Choices
The request has multiple possible responses. Learn when to use 300 Multiple Choices for content negotiation and alternative resource locations.
HTTP Status Codes
303 See Other
Redirect to a different resource using GET. Learn when to use 303 to prevent form resubmission and implement the Post-Redirect-Get pattern.
HTTP Status Codes
307 Temporary Redirect
Temporary redirect that preserves the HTTP method. Learn when to use 307 instead of 302 for method-sensitive redirects.
HTTP Status Codes
308 Permanent Redirect
Permanent redirect that preserves the HTTP method. Learn when to use 308 instead of 301 for method-sensitive permanent redirects.
HTTP Status Codes
402 Payment Required
Reserved for future use in digital payment systems. Learn about this experimental status code and modern payment verification alternatives.
HTTP Status Codes
418 I'm a Teapot: Origin and Real-World Use
418 I'm a teapot comes from the 1998 April Fools RFC 2324. See why RFC 9110 reserves it, why Node and Go keep it, and why some sites return it to bots.
HTTP Status Codes
426 Upgrade Required
The server refuses to perform the request using the current protocol and requires the client to upgrade to a different protocol.
HTTP Status Codes
HTTP 302 Found: Temporary Redirect
Learn what 302 redirect means, when to use temporary vs permanent redirects, and how 302 differs from 301, 307, and 308.
HTTP Headers
Accept Header
Learn how the Accept header tells servers which content types (JSON, HTML, XML) your client can handle. Master content negotiation and quality values.
HTTP Headers
Accept-Language Header
Learn how the Accept-Language header tells servers which languages your client prefers for localized content. Understand language tags and quality values.
HTTP Headers
Connection Header
Learn how the Connection header controls whether HTTP connections stay open (keep-alive) or close after each request. Optimize with persistent connections.
HTTP Headers
Content-Disposition Header
Learn how the Content-Disposition header controls whether content displays inline or downloads as an attachment. Set custom filenames for file downloads.
HTTP Headers
Content-Language Header
Learn how the Content-Language header specifies the natural language(s) of response content. Understand language tags and internationalization best practices.
HTTP Headers
Content-Length
Learn how Content-Length specifies the body size in bytes. Essential for progress indicators, connection management, and chunked transfer decisions.
HTTP Headers
Content-Location Header
Learn how Content-Location indicates an alternate URL for returned content. Useful for content negotiation and identifying canonical resource locations.
HTTP Headers
Date Header
Learn how the Date header specifies when the HTTP message was originated by the server. Understand date formats and its role in caching and logging.
HTTP Headers
Expect Header: 100-continue, curl Delays and 417
Expect: 100-continue asks the server to approve a request before the body is sent. Why curl adds it past 1 MB, the 1s delay, -H "Expect:", and 417.
HTTP Headers
Forwarded
Learn how the Forwarded header preserves original client information (IP, protocol, host) that would otherwise be lost when requests pass through proxies.
HTTP Headers
Host Header
Learn how the Host header specifies the target server domain name and port for HTTP requests. Essential for virtual hosting and routing on shared servers.
HTTP Headers
Location
Learn how the Location header specifies redirect URLs or the location of newly created resources. Essential for 201, 301, 302, and other redirect responses.
HTTP Headers
Referer Header: What It Sends and How to Control It
Referer tells a server which page a request came from. What browsers send under the default strict-origin-when-cross-origin policy, and how to trim it.
HTTP Headers
Refresh Header
Learn how the Refresh header instructs browsers to reload or redirect after a delay. Understand its use cases, limitations, and better alternatives.
HTTP Headers
Retry-After
Learn how the Retry-After header tells clients how long to wait before retrying a request. Understand its use with 503, 429, and 301 status codes.
HTTP Headers
Sec-WebSocket-Accept
Learn how the Sec-WebSocket-Accept header indicates server acceptance of a WebSocket connection upgrade. Understand the handshake process and key validation.
HTTP Headers
Sec-WebSocket-Key
Learn how the Sec-WebSocket-Key header provides a random key for WebSocket handshake validation. Understand the upgrade process and security implications.
HTTP Headers
Transfer-Encoding Header
Learn how the Transfer-Encoding header specifies encoding formats like chunked transfer for streaming responses when content length is unknown beforehand.
HTTP Headers
Upgrade Header
Learn how the Upgrade header requests protocol upgrades to WebSocket, HTTP/2, or other protocols on the same TCP connection. Understand upgrade negotiation.
HTTP Headers
User-Agent Header
Learn how the User-Agent header identifies the client software, browser, or application making HTTP requests. Understand user agent strings and best practices.
HTTP Headers
Via Header
Learn how the Via header tracks the path of HTTP requests through proxies and gateways. Debug routing issues and understand your network infrastructure.
HTTP Headers
Warning Header
Learn about the deprecated Warning header that provided additional status information about message transformations. Understand why it was removed from HTTP.
HTTP Headers
X-Forwarded-For
X-Forwarded-For carries the client IP through proxies and load balancers, but clients can forge it. Trust it safely in nginx, Express, Cloudflare and AWS.
HTTP Headers
X-RateLimit Headers
Learn how X-RateLimit headers inform API clients about rate limits, remaining requests, and reset times. Implement proper rate limiting in your applications.
Cookie Attributes
Domain
Learn how the Domain cookie attribute controls which domains can access cookies. Understand subdomain sharing, security implications, and restrictions.
Cookie Attributes
Expires
Learn how the Expires cookie attribute sets an absolute expiration date. Understand date formats, timezone handling, and when to use Expires vs Max-Age.
Cookie Attributes
Max-Age
Learn how the Max-Age cookie attribute sets expiration in seconds from now. Understand why Max-Age is preferred over Expires for reliable lifetime control.
Cookie Attributes
Path
Learn how the Path cookie attribute restricts which URL paths can receive cookies. Understand path matching rules and how to scope cookies to specific routes.
Guides
Content Negotiation: Accept, Languages and Caches
Implement HTTP content negotiation with Accept headers, q-values, Vary and 406 responses, with Express, Django and nginx examples and language SEO guidance.
Guides
HTTP Sessions and State Management Explained
Learn how to manage user state and sessions in stateless HTTP applications using cookies, tokens, and server-side storage.
Guides
HTTP Status Codes: Full List with Meanings
Every HTTP status code class from 1xx to 5xx with what each code means and when to return it, including the ones you meet most: 200, 301, 304, 404 and 500.
Guides
Request and Response Lifecycle
Learn how HTTP requests travel from browser to server and back. Understand DNS resolution, TCP connections, request/response flow, and the complete lifecycle.
Guides
WebSockets over HTTP: Handshakes, Proxies and Auth
Trace a WebSocket HTTP handshake, verify Sec-WebSocket-Accept, configure nginx, secure browser authentication, and debug idle disconnects and close codes.
Glossary
Content Negotiation
Content negotiation lets one URL serve different formats, languages or encodings based on Accept headers. See q-values, Vary, 406 and the caching traps.
Glossary
HTTP Cookie
Learn what HTTP cookies are and how browsers store small data pieces for websites. Understand cookie attributes, security, and session management.
Glossary
HTTP Header
Learn what HTTP headers are and how they provide metadata about requests and responses. Understand common headers like Content-Type and Authorization.
Glossary
HTTP Method
Learn what HTTP methods are and how they define actions on resources. Understand GET, POST, PUT, DELETE, PATCH, and other methods with examples.
Glossary
HTTP Payload
Learn what HTTP payload means and how message bodies carry data in requests and responses. Understand JSON, form data, and binary payloads.
Glossary
HTTP Request
Learn what an HTTP request is and how clients send messages to servers. Understand request structure, methods, headers, and body components.
Glossary
HTTP Response
Learn what an HTTP response is and how servers reply to client requests. Understand response structure, status codes, headers, and body content.
Glossary
HTTP Session
Learn what HTTP sessions are and how they maintain state across stateless HTTP requests. Understand session cookies, tokens, and server-side storage.
Glossary
HTTP Status Code
Learn what HTTP status codes are and how they indicate request results. Understand 1xx, 2xx, 3xx, 4xx, and 5xx code classes with common examples.
Glossary
Idempotent
Learn what idempotent means in HTTP. Understand why GET, PUT, and DELETE are idempotent, why POST is not, and how idempotency affects API design.
Glossary
MIME Type (Media Type)
A MIME type, or media type, is the type/subtype label in Content-Type that tells clients how to interpret a body. Covers nosniff and blocked-script errors.
Glossary
URI vs URL (and URN)
A URL is a URI that says where a resource lives; a URI only identifies it. See the parts of a URL, what the server never receives and parser pitfalls.

HTTP DELETE Method: Remove Resources

HTTP HEAD Method

HTTP OPTIONS Method

HTTP PATCH Method

HTTP PUT Method: Update Resources

HTTP QUERY Method: Safe Requests With a Body (RFC 10008)

100 Continue

101 Switching Protocols

102 Processing

202 Accepted

203 Non-Authoritative Information

204 No Content

205 Reset Content: What It Does and Browser Support

207 Multi-Status: WebDAV Per-Resource Results

300 Multiple Choices

303 See Other

307 Temporary Redirect

308 Permanent Redirect

402 Payment Required

418 I'm a Teapot: Origin and Real-World Use

426 Upgrade Required

HTTP 302 Found: Temporary Redirect

Accept Header

Accept-Language Header

Connection Header

Content-Disposition Header

Content-Language Header

Content-Length

Content-Location Header

Date Header

Expect Header: 100-continue, curl Delays and 417

Forwarded

Host Header

Location

Referer Header: What It Sends and How to Control It

Refresh Header

Retry-After

Sec-WebSocket-Accept

Sec-WebSocket-Key

Transfer-Encoding Header

Upgrade Header

User-Agent Header

Via Header

Warning Header

X-Forwarded-For

X-RateLimit Headers

Domain

Expires

Max-Age

Path

Content Negotiation: Accept, Languages and Caches

HTTP Sessions and State Management Explained

HTTP Status Codes: Full List with Meanings

Request and Response Lifecycle

WebSockets over HTTP: Handshakes, Proxies and Auth

Content Negotiation

HTTP Cookie

HTTP Header

HTTP Method

HTTP Payload

HTTP Request

HTTP Response

HTTP Session

HTTP Status Code

Idempotent

MIME Type (Media Type)

URI vs URL (and URN)

More Topic Clusters

HTTP Troubleshooting

HTTP Security

HTTP Performance

HTTP Authentication