Basic Concepts, Protocols and Terminology - An Introduction to Network Programming with Java - page 5

A URL (Uniform Resource Locator) is a unique identifi er for any resource located
on the Internet. It has the following structure (in which BNF notation is used):

<protocol>://<hostname>[:<port>][/<pathname>]
[/<fi lename>[#<section>]]
For example:

http://www.oracle.com/technetwork/java/javase/down-
loads/index.html
For a well-known protocol, the port number may be omitted and the default port
number will be assumed. Thus, since the example above specifi es the HTTP proto-
col (the protocol of the Web) and does not specify on which port of the host machine
the service is available, it will be assumed that the service is running on port 80 (the
default port for Web servers). If the fi le name is omitted, then the server sends a
default fi le from the directory specifi ed in the path name. (This default fi le will com-
monly be called index.html or default.html .) The 'section' part of the URL (not often
specifi ed) indicates a named 'anchor' in an HTML document. For example, the
HTML anchor in the tag

<A HREF="#summary">Summary of Report</A>
would be referred to as summary by the section component of the URL.
Since human beings are generally much better at remembering meaningful
strings of characters than they are at remembering long strings of numbers, the
Domain Name System was developed. A domain name , also known as a host name ,
is the user-friendly equivalent of an IP address. In the previous example of a URL,
the domain name was www.oracle.com . The individual parts of a domain name
don't correspond to the individual parts of an IP address. In fact, domain names
don't always have four parts (as IPv4 addresses must have).
Normally, human beings will use domain names in preference to IP addresses,
but they can just as well use the corresponding IP addresses (if they know what they
are!). The Domain Name System provides a mapping between IP addresses and
domain names and is held in a distributed database. The IP address system and the
DNS are governed by ICANN (the Internet Corporation for Assigned Names and
Numbers), which is a non-profi tmaking organisation. When a URL is submitted to
a browser, the DNS automatically converts the domain name part into its numeric IP
equivalent.

1.5 TCP

In common with all modern computer networks, the Internet is a packet-switched
network, which means that messages between computers on the Internet are broken
up into blocks of information called packets , with each packet being handled sepa-
rately and possibly travelling by a completely different route from that of other such