imported>Fbergo: Restored revision 1274395475 by 176.121.250.147 (talk): Rv, incorrect

2025-02-27T16:30:10Z

Restored revision 1274395475 by 176.121.250.147 (talk): Rv, incorrect

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{{Short description|Information at the beginning of data which describes or defines the data}}
In [[information technology]], '''header''' is supplemental [[data]] placed at the beginning of a block of data being stored or transmitted. In [[data transmission]], the data following the header is sometimes called the ''[[Payload (computing)|payload]]'' or ''[[HTML element#body|body]]''.

It is vital that header composition follows a clear and unambiguous [[specification]] or format, to allow for [[parsing]].

==Examples==
* [[E-mail header]]: The text (body) is preceded by header lines indicating sender, recipient, subject, sending time stamp, receiving time stamps of all intermediate and the final [[mail transfer agent]]s, and much more.{{Ref RFC|5322}}{{Ref RFC|6854}}
* Similar headers are used in [[Usenet]] ([[NNTP]]) messages, and [[HTTP header]]s.
* In a [[data packet]] sent via the [[Internet]], the data (payload) are preceded by header information such as the sender's and the recipient's [[IP address]]es, the [[Communications protocol|protocol]] governing the format of the payload and several other formats. The header's format is specified in the [[Internet Protocol]], see [[IP header]].
* In data packets sent by [[wireless communication]], and in [[Disk sector|sector]]s of data stored on [[magnetic media]], typically the header begins with a [[syncword]] to allow the receiver to adapt to analog amplitude and speed variations and for [[frame synchronization]].
* In [[graphics file format]]s, the header might give information about an image's size, resolution, number of colors, and the like.
* In [[archive file format]]s, the file header might serve as a [[fingerprint]] or [[signature]] to identify the specific [[file format]] and corresponding software utility.
* In some [[programming language]]s (for example [[C (programming language)|C]] and [[C++]]) the [[subroutine|functions]] are declared in [[header file]]s.

==See also==
* [[Page footer|Footer]]
* [[Protocol overhead]]
*[[Trailer (computing)]], used in computer networking
* [[Field (computer science)]]

==References==
{{Reflist}}

[[Category:Computer data]]
[[Category:Data transmission]]

Header (computing) - Revision history

imported>Fbergo: Restored revision 1274395475 by 176.121.250.147 (talk): Rv, incorrect