DNS

As you have learned in the internet addressing section, unique computer names are mapped to unique IP-Addresses. The most simple reason fort this was (and still is), that names are much easier to remember than a set of four numbers. In order for this to work, there has to be a simple database, where the information, which name is assigned to which number is stored. In the early years of the internet each system connected to the Internet had such a small file, giving the name and the assigned IP address. Due to the large number of computers connected to the internet, this has become very unhandy. At this time, special computers have been connected to the Internet with the only purpose of carrying such a database and acting as a name server. Please note that this isn't a single computer, rather than a set of computers, all connected together. The name information is stored in a tree. Let's say you want to access a computer called www.uni-linz.ac.at. In order to find the corresponding IP address you would have to ask a name server, carrying IP addresses all the AT computers. First you might get the names and addresses of several servers for AT. Most of the time there is more than one server. One serves as a backup server, if the primary fails. Now you would ask the AT name server where the servers for AC is. Also there might be more than one addresses returned for the above reasons. Then you would ask the server for AC where UNI-LINZ is. And finally you would ask the server for UNI-LINZ about WWW. The result is an IP address used to make the desired connection. All the querying is done automatically and there is no interference required by the user. The only thing you have to do is, specify a name server for your internet connection. This setting often can be made in the TCP/IP profile. If this is not done and you want to access a computer by domain name, then you will get an error message like address not found. Each of the above levels (at, ac, ...) is called a domain. Of course the above description is a very simplified way, but I think it outlines how this system works. There are sophisticated mechanisms for remembering where a certain domain can be found and more. Such a query is processed in a matter of milliseconds (sometimes it might take a few seconds though). A computer name is not just a few words separated by some points. Each single domain has a special meaning. Usually a name contains information about the service, the company name, the type of institution and the country. Let's have a look at the following address: www.uni-linz.ac.at The first domain www is short for World Wide Web. The second domain normally is the name of the company or institution running the computer. Uni-Linz is short for University Linz. The third domain specifies the type of organization. AC is short for academic. Other possible domains are co, which is short for comercial; or, which is short for organization (usually non-profit). The fourth domain defines the country the computer is located in. In this case at stands for Austria. The above example is normally used for computers outside the U.S.A. Computers located in the U.S.A usually have only three domains, where the last domain specifies the type of organization. See domain list for more details and examples.

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