1.5. URLS AND DOMAIN NAMES

19

USER

% telnet www.cs.brandeis.edu 7

Trying 129.64.2.3...

Connected to diamond.cs.brandeis.edu.

Escape character is '^]'.

USER

This is the echo port

This is the echo port

USER

bye bye

bye bye

USER

^]

telnet>

USER

quit

Connection closed.

Figure 1.3: Accessing the echo service on port 7

The HTTP service is one half of the technological foundation of the World

Wide Web. The other half is the HTML language. HTML is an acronym for

Hypertext Markup Language. HTML specifies the layout of webpages and pro 

vides mechanisms for including links to other webpages and to images, sounds,

movies, and other content. In the next Chapter we will provide an introduction

to HTML and some related technology (CSS and XML).

Figure 1.4 gives an example of the use of this service to request the web page

 / cs2a/index.html  from the server  www.cs.brandeis.edu . Observe that the

request specifies the page to access and the response provides quite a bit of

information about the file including its size, its last modification date, its size,

what type of information is in the file, the kind of server that is providing the

service, the locate time at which the page is being served, and some more arcane

information as well.

There are many web browsers that are currently available. The most com 

mon browsers at the moment are Internet Explorer and Netscape, but some of

the lesser known browsers such as Opera and Amaya, provide additional fea 

tures which are not currently supported by the mainstream browsers such as

mathematical and graphical markup processing.

1.5

URLs and Domain Names

As you probably know, all of the content on the World Wide Web can be accessed

by providing its address to a browser. The formal name for a web address is

URL which stands for Universal Resource Locator

1

URLs are our first example

of a formal language. Each URL has several parts, some of which are optional.

Some examples of URLs are:

http://www.brandeis.edu

1

Some people also use URI for Universal Resource Identifier.