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HTTP header

A record sent by clients and servers communicating with each other via the HTTP protocol. The header is a stream of text that may be sent without any content following it or with the content that it describes. There are headers specific to requests and to responses and others that are used for both client and server to describe or query the content or environment. An example of a common request header sent to the server is If-Modified-Since. The server returns the file only if it has been changed since a certain date and time. See HTTP.


(HyperText Transfer Protocol) The communications protocol used to connect to Web servers on the Internet or on a local network (intranet). Its primary function is to establish a connection with the server and send HTML pages back to the user's browser. It is also used to download files from the server either to the browser or to any other requesting application that uses HTTP.

Addresses of websites begin with an http:// prefix; however, Web browsers typically default to the HTTP protocol. For example, typing is the same as typing In fact, only has to be typed in. The browser adds the rest.

With HTTP, the Web page is transmitted without any encryption. However, HTTPS (HTTP Secure) is used to encrypt sensitive data such as credit card and social security numbers (see HTTPS).

A Stateless Connection
HTTP is a "stateless" request/response system. The connection is maintained between client and server only for the immediate request, and the connection is closed. After the HTTP client establishes a TCP connection with the server and sends it a request command, the server sends back its response and closes the connection.

The first version of HTTP caused considerable overhead. Each time a graphics file on the page was requested, a new protocol connection had to be established between the browser and the server. In HTTP Version 1.1, multiple files could be downloaded with the same connection. It also improved caching and made it easier to create virtual hosts (multiple websites on the same server). See HTTP/2, HTTP header and cookie.

Web Server Fundamentals
Web browsers communicate with Web servers via the TCP/IP protocol. The browser sends HTTP requests to the server, which responds by sending back headers (messages) and files (HTML pages, image files, Java applets, etc.). See HTTP header.

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Before/After Your Search Term
HTPCHTTP Live Streaming
HTTP/1.0 404 Object Not FoundHTTP proxy
HTTP/2HTTP request header
HTTP downloadHTTP response header
HTTP Dynamic StreamingHTTP return codes
HTTP error codesHTTP server
HTTP file transferHTTP server codes
HTTP GETHTTP streaming

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