Just a little bit of theory

IP addresses

When two computers are connected each other, they made up a network. As you already know, the Internet is a huge set of different networks connected each other. The Internet is a *public* network. Each computer in such a huge network is reachable by means of a unique IP (Internet Protocol) address. You can imagine an IP address as a street number. Your city is the network, streets are cables and houses are computers. When you have to reach a house that you don't know you must have its address. An address consist of a street and a street number. You can't reach that house if you don't know its street number. Anyway, networks 'street numbers' are a bit odd: 4 digits separated by dots. For example 64.233.167.99 is an IP address. If you run your favorite web browser (Firefox, Internet Explorer or whatever you like) and you type: http://www.google.com, you reach the Google's house. Now, forget 'http://www.google.com' and digit 64.233.167.99 instead... As you can see, things don't change: you can reach the Google's house again. When you type the Google's address, you are asking to your web browser: 'please Firefox, bring me to the Google's house'. As there are millions of 'houses' out of there, IP addresses are assigned by a centralized organization: IANA (Internet Assigned Number Authority). In fact, suppose you wanted to set up your personal web site: you couldn't use the 64.233.167.99 IP address as it belongs to Google. Am I right?

Things are a little more complicated anyway. Imagine a world of numbers: Google is 64.233.167.99, IANA is 192.0.34.162, Yahoo is 68.142.226.47. Imagine your bookmark: 64.233.167.99, 192.0.34.162, 68.142.226.47 and so on. There is no human capable of remembering all those numbers! For this reason, humans developed a tool to help themselves. A kind of 'telephone book'. This kind of telephone book is named DNS (Domain Name Service). What is the DNS? It is a method by which Internet addresses in mnemonic form such as http://www.google.com are converted into the equivalent numeric IP address (in this case: 64.233.167.99). Did you get me? The Google's house is a specific computer whose IP address is 64.233.167.99. The mnemonic address of the Google's house is http://www.google.com. The name of the Google's house is 'Google.com'. The 'name' of the Google's house is a *domain name* actually. It is made up of one 'top level' domain name and one 'second level' domain name. 'Com' is the top level domain name and 'Google' is the second level domain name.

Time ago, IANA assigned 7 different top level domain names: 'com' for commercial organizations, 'edu' for educational organizations, 'gov' for government, 'int' for international organizations, 'mil' for military organizations, net' for network related and 'org' for miscellaneous organizations (usually no profit organizations). Furthermore, there are several two letters top level domains for all countries of the world ('it' for Italy, 'hk' for Hong Kong, 'dk' for Denmark and so on; refer to the ISO3166 document for more). If you want to have your own domain name, you can choose and register it. Once it is registered, nobody but you can use it. There are some organizations that are allowed to register domain names. Such organizations are named 'registrars'. Registrars store domain names inside of their archives. So if you want to set up a web site named 'yourcoolsite.com', you have to register the domain name yourcoolsite. But, wait a moment...how can Firefox know that www.google.com is equivalent to 64.233.167.99? Well, it doesn't know that at all! In fact every time you type www.google.com in your favorite web browser address box, something happens secretly: your web browser asks to a DNS server for a translation of the mnemonic address (www.google.com) to a real numeric IP address (64.233.167.99). The DNS server, is a specialized computer whose task is translating mnemonic addresses to real IP addresses. It queries the registrars' archives to do that. The DNS is a hierarchical system. At the top of it there are 13 universal DNS root servers currently. 10 of them are located in the USA.

One more thing: as said above, the Internet is a huge set of *different* networks connected each other. Notice the word 'different'. There are so many different kinds of networks out of there. There are Macintosh networks, Microsoft networks, SUN networks, IBM networks and so on. Each of them speaks a different language. They couldn't talk each other. Imagine a chinese guy who wants to talk to an Italian guy or a French guy who wants to talk to a Spanish guy...They need a translator. Or they could talk a common language: the english language for example. Well, computer networks use a common language actually: its name is TCP (Transmission Control Protocol). When one computer inside of one network wants to talk to one computer inside of another network, it has to know the other computer address (IP) and it has to use a common language (TCP). So communications among computer networks make use of the TCP/IP protocol.

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