How Internet Works Internet was started in 1960 at U.S. defense department as a network of 4 computers. Yes only 4 computers. It was called ARPAnet. In 1990 it was disbanded and a network came forward with hundreds (200+sites) of site. Today we access internet via a gateway known as ISP. There is a network of computers in America known as internet backbone. It is administrated by NIC network information centre. This network has all the information about internet. It takes care that every computer connected to internet has a different IP address. Everybody has to hire a IP address from internet. We will see what are IP addresses latter. Now our ISPs like sancharnet, VSNL etc. are connected to this internet backbone via a special hardware. A special hardware means the hardware capable of data transfer at very high bandwidth may be 1Gbps(Giga bits not Gigabytes). These ISPs offer internet connectivity to their clients using a dial up modem at 56Kbps. When you connect to the internet you are assigned a 32 bit number called IP address. This IP address is your identity on internet. No two hosts can have same IP addresses at a given point of time. Your ISP hires a number of IP addresses from NIC and assigns to it's users as they connect to the net. If an ISP has 10,000 users it will hire only 1000 IP addresses as all 10,000 people need not be using net at the same time. That is why you can not connect yourself to net sometimes. You access websites using their addresses which have a format. http://www.msdn.com/ here http:// is called protocol, www signifies that the site belongs to world wide web that is you are viewing web pages. msdn is the real name of site where as .com is called domain name. you can not create such names on your own. You have to buy domain names from INTERNIC a part of NIC. These domain names are nothing but an alias for IP addresses. Since human beings can not remember IP addresses which have a format like 18.104.22.168 these IP addresses are given these names. But when you type web address in your browser how do you get the information from that site? How does my request reaches to the server hosting that website? First of all do you know that a website is said to be hosted by a computer called server. This computer is the one that runs a webserver software such as Apache. So we want to know how does our request reaches to msdn.com when we type that address in address bar of our browser (a software that is used to surf net. e.g. Internet Explore). you type http://www.msdn.com/ and your browser contacts your ISP server that you want to connect to that server. Your ISP checks whether it's database has the IP address of msdn.com. If it has it it will send back to the browser else it will ask the internet backbone to find it. Internet backbone surely keeps record of all domain and their IP addresses. Thus IP address travels back to your browser. Now the browser has the IP address it sends request to webserver software asking for the default webpage usually index.htm. Physical position of any server can be very easily found from the structure of IP address so it is not difficult to locate that server. To understand this you should know what is an IP address. Once our request reaches to the webserver software it replies accordingly. Thus we recieve information from that server. Let us see IP addresses in detail. It lies between 0.0.0.0 to 255.255.255.255 It is a 32 bit number divided in 4 fields separated by a dot. These fields have special meanings. Whenever a field contains 0 that means that particular host only. That is why your computer has a loopback address 127.0.0.1. If you have not understood what it means forget you will soon understand. If any field has -1 then it will be a broadcast IP address that is to all the hosts. To understand this concepts perfectly we should see what are the network classes. every IP address is divided into two main parts a network part and host part. Consider you have a network in college A and another network in college B. Assuming they are class A networks one host in college A has IP address 22.214.171.124 In this case all the hosts on that network will have first field (60) in common and no other host in the world will have first field common to that of college A. That is the network in your college is uniquely identified by that number 60. Remaining fields identify the hosts on that particular network. College B must have different first field. It should be other than 60. That is the speciality of class A networks. In class B networks first two fields are for networks and remaining fields represent host numbers. Thus NIC has a clear idea where each host is situated in world.