World Wide Web (WWW)
Proxy Server
Network Address Translator (NAT)
Internet Technologies
What is WWW?
System of interlinked Hypertext documents
Text, Images, Videos, and other multimedia documents navigate
between them via Hyperlinks
Components of WWW
Web Server
Maintains the resources which the user wants to share
These resources are liked to each other to navigate from one another Also Called as HTTP server
HTML
Language to publish the contents
It tells how to display the document and how it should be presented
Hyper Text Transfer Protocol (HTTP)
Language for server and client to communicate
URL
Is a character string that specifies where a known resource is available on
the internet and the mechanism for retrieve it.
The syntax is
scheme://domain:port/path?query_string#fragment_id
Scheme:
Type of the service to be used to access the resource Like http, ftp, mailto, https etc.
Domain or IP Address:
Domain name or the IP address of the web server where the resource is located
Port:
Port number of the destination http server Port number is optional
If port number if omitted port number 80 is assumed (port number 80 is default http
port)
URL Continued
Path
Path of the resource to be fetched or path of the script to be
executed by the server
Query_String
Contains the form data to be processed by the program running
on the server
Fragment ID:
Specifies a part or a position within the overall resource or
document.
When used with HTTP, it usually specifies a section or location
within the page, and the browser may scroll to display that part of the page.
URL Examples
http://google.com:80
Performing an HTTP request to the host at google.com, using
the port number 80
mailto:[email protected]
Start an e-mail composer with the address
[email protected] in the To field
ftp://asmith:[email protected]
http://dcis.uohyd.ernet.in/~askcs/ca502.html
http://www.xyz.com/cgi-bin/xyz.pl?Roll=1234&Sex=M
Form data is provided as input to the script xyz.pl
HTTP Basics
Protocol for client/server communication
Client sends a request message, server replies with response
message
First client establishes a socket connection with the server
and then the HTTP request has to be sent
Stateless
HTTP Request commands
GET – Retrieve the document specified by the URL
HEAD – Retrieve the header information about a document
specified by the URL
POST – Give information to the server
PUT – Store specified document under the given URL DELETE – Remove the document specified by the URL
HTTP Request methods
Basic Syntax of the HTTP request
an initial line,
zero or more header lines,
a blank line (i.e. a CRLF by itself), and
an optional message body (e.g. a file, or query data, or query output).
GET /path/to/file/index.html HTTP/1.0
GET /path/script.cgi?field1=value1&field2=value2 HTTP/1.0
Initial Response Line (Status Line)
The initial response line, called the status line, also has three parts separated
by spaces: the HTTP version, a response status code that gives the result of the
request, and an English reason phrase describing the status code. Typical status
lines are:
HTTP/1.0 200 OK or HTTP/1.0 404 Not Found
HTTP response codes
The status code is a three-digit integer, and the first digit
identifies the general category of response: 1xx indicates an informational message only 2xx indicates success of some kind
3xx redirects the client to another URL 4xx indicates an error on the client's part 5xx indicates an error on the server's part
The most common status codes
200 OK - The request succeeded, and the resulting resource is
returned in the message body.
Some HTTP exchanges with GET
To retrieve the file at the URL http://www.somehost.com/path/file.html
Client request
GET /path/file.html HTTP/1.0 From: [email protected] User-Agent: HTTPTool/1.0 [blank line here]
Server response
HTTP/1.0 200 OK
Date: Fri, 31 Dec 1999 23:59:59 GMT Content-Type: text/html
Content-Length: 1354 <html>
<body>
<h1>Happy New Millennium!</h1> (more file contents) . . .
</body> </html>
HEAD HTTP Request method
Similar to GET but it requests only the document header
information only
Document contents are not downloaded from the server This is useful to check the characteristics of the resource
without downloading it
POST HTTP Request
POST is used when data to be sent to the server which is
processed by some program at the server
This is used when POST action method is used in the form
POST /path/script.cgi HTTP/1.0 From: [email protected]
User-Agent: HTTPTool/1.0
Content-Type: application/x-www-form-urlencoded Content-Length: 32
HTTP Proxy Server
An HTTP proxy is a program that acts as an intermediary between a
client and a server
It receives requests from clients, and forwards those requests to
the intended servers
The responses pass back through it in the same way It acts both as a server and client
Proxy can also be used a network firewall
When proxy is present clients has to provide complete URL of the
resource
GET http://www.somehost.com/path/file.html HTTP/1.0
Otherwise proxy does not have any information about the domain
Network Address Translator (NAT)
IP addresses are limited in number.
Not always possible to assign a public IP address to each
system on the network
NAT allows single device (router) to act as an agent between
the internet (public network) and the local (private) network.
Only single IP address is required to represent group of local
computers
Types of NAT
Static NAT
Mapping an unregistered IP address to a registered IP address on a
one-to-one basis
Not much of use except some filtering the contents
Dynamic NAT
Maps an unregistered IP address to a registered IP address from a group of IP
addresses
Overloading
Maps multiple unregistered IP address to a single registered IP address using
different port numbers (Port Address Translation)
Stub Domain
It is a LAN that uses IP addresses internally (private IP addresses).
Most of the traffic is local
A snub domain can include both registered and unregistered IP addresses
