Domain Name System

Domain Name System (DNS), in computer communications, a method of translating Internet addresses so that computers connected in the Internet can find each other. A DNS server translates a numerical address assigned to a computer (such as 207.46.228.91) into a sequence of words, and vice versa. A DNS name, written in lowercase letters with words separated by periods, takes the form of username@computer.zonename (for example: president@whitehouse.gov). Username is the name or account number used to log on. The hostname (Whitehouse in the example above) is the name of the computer or Internet provider; it may consist of several parts. Zonename indicates the type of organization. Common zone names include com (commercial organization), edu (educational), gov (government), and net (networking organization).

World Wide Web

World Wide Web (WWW), computer-based network of information resources that a user can move through by using links from one document to another. The information on the World Wide Web is spread over computers all over the world. The World Wide Web is often referred to simply as “the Web.”

The Web has become a very popular resource since it first became possible to view images and other multimedia on the Internet, a worldwide network of computers, in 1993. The Web offers a place where companies, institutions, and individuals can display information about their products, research, or their lives. Anyone with access to a computer connected to the Web can view most of that information. A small percentage of information on the Web is only accessible to subscribers or other authorized users. The Web has become a forum for many groups and a marketplace for many companies. Museums, libraries, government agencies, and schools make the Web a valuable learning and research tool by posting data and research. The Web also carries information in a wide spectrum of formats. Users can read text, view pictures, listen to sounds, and even explore interactive virtual environments on the Web.

Like all computer networks, the Web connects two types of computers–clients and servers—using a standard set of rules for communication between the. The server computers store the information resources that make up the Web and Web users use client computers to access the resources. A computer-based network may be a public network—such as the worldwide Internet—or a private network, such as a company’s intranet. The Web is part of the Internet. The Internet also encompasses other methods of linking computers, such as Telnet, File Transfer Protocol, and Gopher, but the Web has quickly become the most widely used part of the Internet. It differs from the other parts of the Internet in the rules that computers use to talk to each other and in the accessibility of information other than text. It is much more difficult to view pictures or other multimedia files with methods other than the Web.

Enabling client computers to display Web pages with pictures and other media was made possible by the introduction of a type of software called a browser. Each Web document contains coded information about what is on the page, how the page should look, and to which other sites the document links. The browser on the client’s computer reads this information and uses it to display the page on the client’s screen. Almost every Web page or Web document includes links, called hyperlinks, to other Web sites. Hyperlinks are a defining feature of the Web—they allow users to travel between Web documents without following a specific order or hierarchy.

HTTP(Hypertext Transfer Protocol)

The Hypertext Transfer Protocol (HTTP) is an application-level protocol with the lightness and speed necessary for distributed, collaborative, hypermedia information systems. HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification reflects common usage of the protocol referred too as "HTTP/1.0". This specification describes the features that seem to be consistently implemented in most HTTP/1.0 clients and servers. The specification is split into two sections. Those features of HTTP for which implementations are usually consistent are described in the main body of this document.

Practical information systems require more functionality than simple retrieval, including search, front-end update, and annotation. HTTP allows an open-ended set of methods to be used to indicate the purpose of a request. It builds on the discipline of reference   provided by the Uniform Resource Identifier (URI), as a location URL or name URN, for  indicating the resource on which a method is to be applied. Messages are passed in a format similar to that used by Internet Mail and the Multipurpose Internet Mail Extensions (MIME).

HTTP is also used as a generic protocol for communication between user agents and proxies/gateways to other Internet protocols, such as SMTP, NNTP, FTP, Gopher, and WAIS, allowing basic hypermedia access to resources available from diverse applications and simplifying the implementation of user agents.

How Internet Works

The key to how the Internet works lies in understanding what it is: The Internet is a network of networks. It is not an easily defined single object.

The Internet is the world's largest distributed system; it was designed and engineered for redundancy (it has an abundance of routes and connections) and resilience (it easily recovers from a mishap). The Internet is not a single company or a group of companies, nor even a single network. It is a worldwide mesh or matrix of hundreds of thousands of networks, owned and operated by hundreds of thousands of people in hundreds of countries, all interconnected by about 8,000 ISPs (Internet Service Providers). No single organization controls the Internet; not the U.N.; not the biggest ISPs; and the Internet has long since outgrown control by the U.S. government.

The Internet is different from other major services. Electricity tends to be provided by a single company in each geographical area. The ``last mile'' of telephone service to the customer is usually owned by a single company. But in general there is more than one Internet provider in any locale, and there are usually many paths from a local provider in one area to a provider in another area.

When you, the user, look at a web page through the Internet, many things happen along the way. There are various ways to get from your house or office through the ``last mile'' to the Internet: modem dialup, ISDN, DSL, cable modem, wireless, leased line, etc. These various technical methods may provide speeds anywhere from very slow (a few hundred bits per second) to very fast (billions of bits per second). All these access methods are onramps to the information superhighway.

In order to transmit text or pictures, your data is chopped up into small packets which are routed through the Internet. But first they have to go from you to your local ISP, or the equivalent piece of the Internet inside your organization (an intranet). This local ISP is a possible point of failure. If something goes wrong at your local ISP, it may look to you like the Internet is broken. It's not. Only one small piece of it is broken. The rest of the Internet, with its portals and stock portfolios and shops and reams of scientific data and plethora of information and people on it will not break because one ISP does.

To reach a web server, your local ISP sends your packets of data to another ISP, which may send them to another ISP, or through an Exchange Point or a National point access (NAP) or local point access (LAP) to get to another ISP. Thus your packets pass through a chain of ISPs through nodal points to reach their destination. Your packets may pass through fiber optic cables in the ground, satellites in the sky, undersea cables, or radio links. They may travel at speeds including T-1 (1.544 Mbps), T-3 (45Mbps), or faster (or slower). The internet Protocol (IP) ties all of those links together, enabling your packets travel through the Internet.

Eventually your packets arrive at the web server, and the web server sends responses back along a similar path (almost definitely not the same one). Any of these Internet providers can have problems (congestion, broken link, power outage, broken computer, etc.), which may cause the web server to seem slow or unresponsive to you. But the web server is broken only if the web server is actually broken. Problems in intervening parts of the Internet do not break the web server, which may well be accessible to other people, and may become accessible to you as soon as the various Internet providers route your traffic around problems.

Much rerouting in the Internet is dynamic, and happens automatically. (Imagine you are driving up the California coast and come to a sign that says that there has been a mudslide. You drive inland, north on another road, perhaps rejoining the coastal highway again. You have changed your route dynamically.) Some rerouting isn't automatic. In particular, the biggest ISPs, frequently called backbones, cover vast geographical areas and carry large proportions of the Internet's traffic. A failure in a backbone or in one of the major interconnection points between them can affect many Internet users. And such a problem may take some time to be resolved, as the biggest ISPs often prefer to manually examine changes in major routes before implementing them.

But longstanding observation of the Internet indicates that ``some time'' is normally at most few hours, even in the face of the biggest problems.

Internet providers use the same methods for routing packets for electronic mail or file transfers or remote login or voice or video. People tend to be quicker to notice slowness in accessing web pages, so we have used accessing a web server as an example.

There are other key pieces of the Internet, most notably the root name servers. Nameservers translate domain names, such as www.ripe.net, into the IP addresses, such as 193.0.0.195, that are used by the Internet protocols in carrying your packets through the Internet. The root nameservers handle the most basic part of that translation, which is finding nameservers for the top level domains (TLDs), such as NET, COM, ORG, EDU, GOV, FR (France), JP (Japan), AU (Australia), or PE (Peru). The root nameservers are widely spaced in both geography and in Internet topology, so that a failure in one cannot readily affect another. The root nameserver operators have also cooperated in extensively testing their software, hardware, and capacities, and they all know how to reach each other in case they perceive problems.

The rest of the Domain Name Service (DNS) is distributed among hundreds of thousands of nameservers for the various domains. For example, there are nameservers for ORG, and then there are nameservers for MIDS.ORG. Every domain is supposed to have at least two independent nameservers, and most do (another instance of redundancy). In any case, a failure in a single nameserver may make a particular domain temporarily inaccessible, but it will not affect the Internet at large.

The decentralization of the Internet is one of its biggest advantages and one of its most basic features, designed into its protocols from the beginning and tested in practice over many years. If one piece breaks, that doesn't mean the Internet is broken. And decentralization requires cooperation, so the various ISPs and IXes and the like are accustomed to cooperating with one another to fix and prevent problems. It is this decentralization and cooperation that has permitted the Internet to grow faster for longer than any other technological phenomenon in history. It is important for you, the user, to understand how decentralization makes the Internet work, so that you will know that the Internet is actually very hard to break.

High-level Language

High-level language is the language, which is very close human language and mathematical notation. Machine cannot directly understand the high level language, therefore must be translated into machine language before the instructions are executed. The program written by the programmer in High-level language is called a source program. The program, which is obtained after being converted into machine language, is called object program. Translation is done by a program i.e., a compiler or interpreter. Interpreter translates one statement at a time where as compiler reads the entire program first and then translates it into machine code. Example of high-level language: FORTRAN, Pascal etc.

Advantage of high-level language:

• Easier to learn and understand.
•  Less time to write program.
• It provides better documentation.
• It is machine independent.
• Easier to maintain.

Disadvantage of high-level language

• It takes more time to execute the program.
• Inefficient object code.
• Difficult in debugging if proper diagnosis is not available.
• Inability to perform all operations because cannot manipulate bits and registers.

Assembly Language

Assembly language is the language that permits the use of mnemonics for each instructing that the machine can do. The word mnemonic refers to memory aid. The program written in assembly language must be transmitted to machine understandable form, which is done by assembler because computer cannot understand mnemonics codes(alphabetic codes) such as ADD for addition, SUB for subtraction etc. The program written by the programmer in assembly language is called a source program. The program, which is obtained after being converted into machine language, is called object program. It is also machine-oriented language ie. Designed for specific make and model of  a computer. An assembly language program would look like the one below.

LOAD A

MULT B

ADD C

STORE X

Advantages

• Simple to understand and use in comparative to machine level language.
•  Less time is required  to write the program in comparative to machine level language.
• Program debugging is easier in comparative to machine level language.

Disadvantage

• It is machine dependent i.e. each design of machine has a different assembly language.
• Assembly level language is too difficult to understand than the high level language.
• Coding in this language is time-consuming.