DSN stands for Domain Name System . Let’s see what it means and what it does.

We human beings can be identified in many ways. For example, we can be identified by the names that appear on our birth certificates. We can be identifies by our social security numbers. We can be identified by our driver’s license number. Although each of these identifiers can be used to identify people, within a given context one identifier may be more appropriate than another. For example, the computer at the IRS (the infamous tax-collecting agency in the United States) prefer to user fixed-length social security numbers rather than birth certificate names. On the other hand, ordinary people prefer the more mnemonic birth certificate names rather than social security numbers. (Indeed, you can imagine saying , “Hi. My name is 132-67-9875. Please meet my husband, 178-87-1146.”)

Just as humans can be identified in many ways, so too can Internet hosts. One identifier for a host is its hostname. Hostnames –such as cnn.com, www.yahoo.com – are mnemonic and are therefore appreciated by humans. However, hostnames provide little, if any, information about the location within the Internet of the host. (A hostname such as www.eurecom.fr, which ends with the country code .fr, tells us that the host if probably in France, but doesn’t say much more). Furthermore, because hostnames can consist of variable length alphanumeric characters, they would be difficult to process by routers. For these reasons, hosts are also identified by so-called IP addresses.

We discuss IP addresses in some detail in Module 4, but it is useful to say a few brief words about them now. An IP address consists of four bytes and has a rigid hierarchical structure. An IP address looks like, where each period separates one of the bytes expressed in decimal notation from 0 to 255. An IP address is hierarchical because as we scan the address from left to right, we obtain more and more specific information about where the host is located in the Internet (that is, within which network, in the network of networks). Similarly, when we scan a postal address from bottom to top, we obtain more and more specific information about where the address is located.