Client, Server

Two often used terms in the context of the Internet are client and
server. A server is a program installed on a certain computer on the
Internet which provides some kind of data or service. A client is a
program running on computers of a local networks. It enables to
contact servers on the Internet and to use their services.
To access a server the client needs information about where to find
the server and what kind of data or service it offers. This information
is generally encoded in a Uniform Resource Locator (URL) which has the
following syntactical structure:

<protocol>://<host>/<path>

where <protocol> denotes a protocol (like http, ftp, news, ... ),
<host> indicates the address of a computer where a server
is installed and <path> names a document or program.
Consider for example the URL:

http://www.techfak.uni-bielefeld.de/
~afaisal/LEH/WWWDESIG/index.html

which denotes the file ~afaisal/LEH/WWWDESIG/index.html
provided by a HTTP server at www.techfak.uni-bielefeld.de .

WWW

The World Wide Web (WWW) is among the most popular applications
of the Internet, because it facilitates to access much of all the
information available.
WWW-clients or browsers are programs used to retrieve information
from all over the world. Since on the Internet information can
manifest in various forms like texts, images, image-sequences,
sound, music, ... these programs should be as flexible as possible,
i.e. they should be able to deal with all these kinds of information.
Two well known browsers are MOSAIC and Netscape .
NSCA is a widespread server, a program used to serve information on the World Wide Web.

Here is an example to illustrate how browsers and servers interact:

• A client asks for http:://host/file.html
• The HTTP-server installed on host searches for the document file.html
• If the document exists, the server determines its MIME-type (this is the Multipurpose Internet Mail Extension a flag which characterises the document's type (e.g. text/plain or image/gif ))
• The server sends MIME-type and document to the client
• If the client cannot display documents of the current MIME-type, an external viewer must be started. This is a program able to handle a document of the given MIME-type (e.g. most browsers cannot display image-sequences (videos), thus if a document of MIME-type video/mpeg was requested, the program mpeg_play is used to depict the document.)

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