Information at the beginning of data which describes or defines the data
Ininformation technology,header is supplementaldata placed at the beginning of a block of data being stored or transmitted. Indata transmission, the data following the header is sometimes called thepayload orbody.
It is vital that header composition follows a clear and unambiguousspecification or format, to allow forparsing.
- 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 finalmail transfer agents, and much more.[1][2]
- Similar headers are used inUsenet (NNTP) messages, andHTTP headers.
- In adata packet sent via theInternet, the data (payload) are preceded by header information such as the sender's and the recipient'sIP addresses, theprotocol governing the format of the payload and several other formats. The header's format is specified in theInternet Protocol, seeIP header.
- In data packets sent bywireless communication, and insectors of data stored onmagnetic media, typically the header begins with asyncword to allow the receiver to adapt to analog amplitude and speed variations and forframe synchronization.
- Ingraphics file formats, the header might give information about an image's size, resolution, number of colors, and the like.
- Inarchive file formats, the file header might serve as afingerprint orsignature to identify the specificfile format and corresponding software utility.
- In someprogramming languages (for exampleC andC++) thefunctions are declared inheader files.
