The above example presents quite a number of URLs. For instance the URL:
http://www.cs.ucl.ac.uk/misc/uk/intro.htmlAs we stated above, this says that the data called `` /misc/uk/intro.html'' can be retrieved from the server running on a computer called `` www.cs.ucl.ac.uk'' using http which is the HyperText Transfer Protocol.
This could equally well say:
http://www.cs.ucl.ac.uk:80/misc/uk/intro.htmlThe number 80 here is the TCP port on the machine www.cs.ucl.ac.uk 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:
ftp://cs.ucl.ac.uk/mice/indexsays 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: