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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Header (computing)</span></span>
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<p>In <a href="Information_technology" title="Information technology">information technology</a>, <b>header</b> is supplemental <a href="Data" title="Data">data</a> placed at the beginning of a block of data being stored or transmitted. In <a href="Data_transmission" class="mw-redirect" title="Data transmission">data transmission</a>, the data following the header is sometimes called the <i><a href="Payload_(computing)" title="Payload (computing)">payload</a></i> or <i><a href="HTML_element#body" title="HTML element">body</a></i>.
</p><p>It is vital that header composition follows a clear and unambiguous <a href="Specification" class="mw-redirect" title="Specification">specification</a> or format, to allow for <a href="Parsing" title="Parsing">parsing</a>.
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<div class="mw-heading mw-heading2"><h2 id="Examples">Examples</h2></div>
<ul><li><a href="E-mail_header" class="mw-redirect" title="E-mail header">E-mail header</a>: The text (body) is preceded by header lines indicating sender, recipient, subject, sending time stamp, receiving time stamps of all intermediate and the final <a href="Mail_transfer_agent" class="mw-redirect" title="Mail transfer agent">mail transfer agents</a>, and much more.<sup id="cite_ref-rfc5322_1-0" class="reference"><a href="#cite_note-rfc5322-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-rfc6854_2-0" class="reference"><a href="#cite_note-rfc6854-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup></li>
<li>Similar headers are used in <a href="Usenet" title="Usenet">Usenet</a> (<a href="NNTP" class="mw-redirect" title="NNTP">NNTP</a>) messages, and <a href="HTTP_header" class="mw-redirect" title="HTTP header">HTTP headers</a>.</li>
<li>In a <a href="Data_packet" class="mw-redirect" title="Data packet">data packet</a> sent via the <a href="Internet" title="Internet">Internet</a>, the data (payload) are preceded by header information such as the sender's and the recipient's <a href="IP_address" title="IP address">IP addresses</a>, the <a href="Communications_protocol" class="mw-redirect" title="Communications protocol">protocol</a> governing the format of the payload and several other formats. The header's format is specified in the <a href="Internet_Protocol" title="Internet Protocol">Internet Protocol</a>, see <a href="IP_header" title="IP header">IP header</a>.</li>
<li>In data packets sent by <a href="Wireless_communication" class="mw-redirect" title="Wireless communication">wireless communication</a>, and in <a href="Disk_sector" title="Disk sector">sectors</a> of data stored on <a href="Magnetic_media" class="mw-redirect" title="Magnetic media">magnetic media</a>, typically the header begins with a <a href="Syncword" title="Syncword">syncword</a> to allow the receiver to adapt to analog amplitude and speed variations and for <a href="Frame_synchronization" title="Frame synchronization">frame synchronization</a>.</li>
<li>In <a href="Graphics_file_format" class="mw-redirect" title="Graphics file format">graphics file formats</a>, the header might give information about an image's size, resolution, number of colors, and the like.</li>
<li>In <a href="Archive_file_format" class="mw-redirect" title="Archive file format">archive file formats</a>, the file header might serve as a <a href="Fingerprint" title="Fingerprint">fingerprint</a> or <a href="Signature" title="Signature">signature</a> to identify the specific <a href="File_format" title="File format">file format</a> and corresponding software utility.</li>
<li>In some <a href="Programming_language" title="Programming language">programming languages</a> (for example <a href="C_(programming_language)" title="C (programming language)">C</a> and <a href="C%2B%2B" title="C++">C++</a>) the <a href="Subroutine" class="mw-redirect" title="Subroutine">functions</a> are declared in <a href="Header_file" class="mw-redirect" title="Header file">header files</a>.</li></ul>
<div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2></div>
<ul><li><a href="Page_footer" title="Page footer">Footer</a></li>
<li><a href="Protocol_overhead" class="mw-redirect" title="Protocol overhead">Protocol overhead</a></li>
<li><a href="Trailer_(computing)" title="Trailer (computing)">Trailer (computing)</a>, used in computer networking</li>
<li><a href="Field_(computer_science)" title="Field (computer science)">Field (computer science)</a></li></ul>
<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
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<li id="cite_note-rfc5322-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-rfc5322_1-0">^</a></b></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">
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</style><cite id="CITEREFP._Resnick2008" class="citation cs1">P. Resnick, ed. (October 2008). <a rel="nofollow" class="external text" href="https://datatracker.ietf.org/doc/html/rfc5322"><i>Internet Message Format</i></a>. <a href="Internet_Engineering_Task_Force" title="Internet Engineering Task Force">IETF</a> Network Working Group. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://doi.org/10.17487%2FRFC5322">10.17487/RFC5322</a></span>. <a href="Request_for_Comments" title="Request for Comments">RFC</a> <a rel="nofollow" class="external text" href="https://datatracker.ietf.org/doc/html/rfc5322">5322</a>.</cite> <i>Draft Standard.</i> Obsoletes RFC <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc2822">2822</a>. Updates RFC <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc4021">4021</a>. Updated by RFC <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc6854">6854</a>.
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<li id="cite_note-rfc6854-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-rfc6854_2-0">^</a></b></span> <span class="reference-text"><cite id="CITEREFB._Leiba2013" class="citation cs1">B. Leiba (March 2013). <a rel="nofollow" class="external text" href="https://datatracker.ietf.org/doc/html/rfc6854"><i>Update to Internet Message Format to Allow Group Syntax in the "From:" and "Sender:" Header Fields</i></a>. <a href="Internet_Engineering_Task_Force" title="Internet Engineering Task Force">Internet Engineering Task Force</a>. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://doi.org/10.17487%2FRFC6854">10.17487/RFC6854</a></span>. <a href="ISSN_(identifier)" class="mw-redirect" title="ISSN (identifier)">ISSN</a> <a rel="nofollow" class="external text" href="https://search.worldcat.org/issn/2070-1721">2070-1721</a>. <a href="Request_for_Comments" title="Request for Comments">RFC</a> <a rel="nofollow" class="external text" href="https://datatracker.ietf.org/doc/html/rfc6854">6854</a>.</cite> <i>Proposed Standard.</i> Updates RFC <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc5322">5322</a>.
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