Protocols

URLs are used in a number of circumstances, to retrieve data using many techniques for many purposes. For example, they are used for e-mail, for file transfer and for USENET news. The first part of the URL therefore gives an indication of the scheme it belongs to. The scheme, or protocol, is separated from the rest of the URL using a colon:

					protocol:scheme-specific-part
				

The protocols for the uses given above are 'mailto:', 'ftp:' (File Transport Protocol) and 'news:':

mailto:scheme-specific-part

ftp:scheme-specific-part

news:scheme-specific-part
				

The syntax of the remainder of the URL depends on the requirements and characteristics of the scheme involved. Here, the discussion concentrates on the file URL scheme and HTTP URL scheme, as the former is used to locate files elsewhere on the local computer system or network, and the latter is the usual method for retrieving information from anywhere on the Internet. They are both regularly used in XML to access entities (particularly DTDs), XML Schemas and stylesheets.