Transcript THE WEB, Chapter 4, 7 and 8

THE WEB
Monica Stoica
Background Information
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HTTP stands for Hypertext Transfer Protocol
FTP stands for File Transfer Protocol
Html stands for hypertext markup language. Html
is the extension used for web page files. Some
computers do not allow more than 3 characters for
extensions, so the extension becomes htm.
Mail addresses are not case sensitive, but URLs
are, especially if the web page resides on an Unix
server.
Introduction to URLs
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URL stands for Uniform
Resource Locator.
Example of URLs:
http://www.uark.edu,
mailto:[email protected],
ftp://ftp.microsoft.com/Products/
msmq/demos.zip
URLs are composed of the
addressing scheme (http://,
mailto:, ftp://) and the
hostname.
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If the left most part of an URL is http://, the
server is a World Wide Web server.
If the leftmost part of an URL is ftp://, the
server is an anonymous FTP server that
allows people to download programs and
other files for free.
There are also mail servers which provide
email services and have names beginning
with mail://.
Other types of addressing schemes:
 news://
 file://
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A scheme identifies the type of
resource.
Most of the URLs use http scheme.
Since http is a protocol used to
transfer Web pages data, the
URLs that use the http scheme
point to Web pages, and not to
anonymous ftp files, or mail
addresses.
Examples of Hostnames
 www.bu.edu
 ftp.xoom.com
 mail.pacbell.net
 www.royal.gov.uk
The Rightmost Part of
the Hostname
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The right most part of the hostname is called
the top-level domain. There are 2 types of toplevel domains:
 ORGANIZATIONAL
(com for commercial
organizations, edu for education, gov for
federal government, net for network, mil for
military) and
 GEOGRAPHICAL DOMAINS (uk, usa, ro).
Domains
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A DOMAIN is a set of hostnames that have the
rightmost part of their names in common. For
example all the hostnames that end in edu belong
to the edu domain, and all the hostnames ending in
uk belong to the uk domain.
When the hostnames have the two rightmost parts
of their names in common, they belong to the same
Second-Level Domain. For example these
following hostnames belong to the same secondlevel domain pacbell.net:
 pacbell.net
 mail.pacbell.net
 www.pacbell.net
 news.pacbell.net
DNS
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All the hostnames (like www.bu.edu) on
the Internet are part of a system called
DNS, the domain name system.
DNS allows you to give a unique name
to each computer on the Internet.
Those names are the hostnames.
IP Numbers
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The computers on the Internet are given a
unique number and the Net uses these
numbers, not the hostnames. These
numbers are called IP ADDRESSES or IP
NUMBERS.
Example: If I want to visit a site
(www.royal.gov.uk), my browser has to find
the corresponding IP number for this
address. My browser calls upon DNS which
translates the hostname into the IP address
(193.32.28.6 in this case).
All IP addresses have the same structure: four
numbers separated by dots.
How DNS Works
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Each organization manages its own hostnames, IP
numbers and sub domains.
Each organization should have 2 computers (one
as a backup), called Name Servers, to provide
addressing information for all the hostnames in its
domain.
At various places on the Net there are a number
of special computers called ROOT NAME
SERVERS. Each root name server maintains a list
of the name servers that handle top-level domains
(edu, com, uk). Each of these name servers
maintains a list of other name servers that handle
the second-level domain and so on.
Example of How DNS Works
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If I was looking for a web site (www.cnn.com),
the DNS contacts the root server then
contacts the com name server and gets the IP
address of the cnn.com name server. Then
the DNS server contacts the cnn.com name
server and gets the IP address of
www.cnn.com and the index.html to be
displayed.
DNS is a good program, but it requires an
amount of time to find an address. This is
why your browser keeps in memory (cache)
all the recently used URLs in case you need
to revisit them.
URL Abbreviations
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Most web servers follow a rule which says
that if a URL specifies a directory but no
file name, the server will automatically
look for a file with a specific name. Some
servers look for a file named index.html,
some for default.html.
If you type a URL that does not have a
scheme at the beginning, your browser
assumes that the URL points to a Web
page and inserts for you http://