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Uniform Resource Locators (URLs)

The above example presents quite a number of URLs. For instance the URL:
As we stated above, this says that the data called `` /misc/uk/intro.html'' can be retrieved from the server running on a computer called ``'' using http which is the HyperText Transfer Protocol.

This could equally well say:
The number 80 here is the TCP port on the machine that the WWW server is listening on. TCP ports are a way that several different kinds of server can all listen on the same machine without getting confused about which server the connection is being made to (think about lots of letter boxes in an apartment block). Port 80 is the default port for the HyperText Transfer Protocol, so if you don't say which port to connect to, Mosaic and the other WWW browsers will all assume you mean port 80. See chapter 5 for more details about server ports and why you might sometimes run a server on a different port.

URLs don't just have to specify that you use HTTP. For instance the URL:
says that to get this information, contact the ftp server running on

Most WWW browsers know how to talk to ftp servers too, so they can set up an ftp connection, and request `` /mice/index'' using the much older File Transfer Protocol.

One of the biggest plus points for Mosaic and other WWW browsers is that they are multiprotocol clients - that is they know about quite a number of different protocols, and so they can contact a number of different types of servers for information. If the information is out there on the Internet, no matter what type of server it's on, there is almost certainly a way for a WWW browser to get it. The URL tells the browser what type of server the data resides on, and thus how to go about getting it.

Protocols that WWW browsers know about include:

HyperText Transfer Protocol
File Transfer Protocol
the menu based information system predating WWW
Wide Area Information System - an information system allowing complex searching of databases
the protocol that allows you to log in to remote systems.
the indexing system that allows you to find out what information is stored where on ftp servers.

This book is primarily about the World Wide Web - to cover all of these protocols in detail would take a book several thousand pages thick, so when talking about protocols, we shall concentrate only on the native protocol spoken by WWW servers - HTTP. For more information on these other protocols and services, so chapter 2.

next up previous contents
Next: An Introduction to Up: The World Wide Previous: Beneath the Surf

Jon Crowcroft
Wed May 10 11:46:29 BST 1995